When U.S. immigration officers in New York City whisked away Ishaq Farhan as he stepped off an incoming international flight in May 2000, his Jordanian diplomatic passport was no help to him. Federal agents questioned him for hours before barring his entry into the country. Then they made him pay for the flight back to Jordan.

The U.S. Embassy in Jordan lost no time making amends to Farhan, a leading opposition politician who has been closely affiliated with the Muslim Brotherhood, a worldwide movement opposed to Western influences. A State Department official visited his home, issued him an immediate visa and passed on the United States' "deep regret for the difficulties Dr. Farhan experienced."

The episode demonstrates the U.S. government's dilemma. Some federal agents worry that the Muslim Brotherhood has dangerous links to terrorism. But some U.S. diplomats and intelligence officials believe its influence offers an opportunity for political engagement that could help isolate violent jihadists.

"It is the preeminent movement in the Muslim world," said Graham E. Fuller, a former CIA official specializing in the Middle East. "It's something we can work with." Demonizing the Brotherhood "would be foolhardy in the extreme," he warned.

The Brotherhood's history and the challenges it poses to U.S. officials illustrate the complexity of the political front in the campaign against terrorism three years after the attacks of Sept. 11, 2001. FBI agents and financial investigators probe the group for terrorist ties and legal violations, while diplomats simultaneously discuss strategies for co-opting at least its moderate wings. In both sectors of the U.S. government, the Brotherhood often remains a mystery.

The Brotherhood -- or al-Ikhwan al-Muslimun, as it is known in Arabic -- is a sprawling and secretive society with followers in more than 70 countries. It is dedicated to creating an Islamic civilization that harks back to the caliphates of the 7th and 8th centuries, one that would segregate women from public life and scorn nonbelievers.

In some nations -- Egypt, Algeria, Syria, Sudan -- the Brotherhood has fomented Islamic revolution. In the Palestinian territories, the Brotherhood created the Islamic Resistance Movement, or Hamas, which has become known for its suicide bombings of Israelis. Yet it is also a sophisticated and diverse organization that appeals to many Muslims worldwide and sometimes advocates peaceful persuasion, not violent revolt. Some of its supporters went on to help found al Qaeda, while others launched one of the largest college student groups in the United States.

For decades, the Brotherhood enjoyed the support of the government of Saudi Arabia and its oil billions, which helped the group expand in the United States.

Past and present Muslim Brotherhood supporters make up the U.S. Islamic community's most organized force. They run hundreds of mosques and dozens of businesses engaging in ventures such as real estate development and banking. They also helped set up some of the leading American Islamic organizations that defend the rights of Muslims, promote Muslim civic activism and seek to spread Islam.

For years federal agents paid little heed to the Brotherhood, but after Sept. 11 they noticed that many leads went back to the Brotherhood. "We see some sort of nexus, direct or indirect, to the Brotherhood, in ongoing cases," said Dennis Lormel, until recently a top FBI counterterrorism official.

The architect of the Sept. 11 strikes, Khalid Sheik Mohammed, told U.S. interrogators that he was drawn to violent jihad after joining the Brotherhood in Kuwait at age 16 and attending its desert youth camps, according to the report released in July by the national commission that investigated the attacks.

Brotherhood radicals in Germany and Spain are suspected of organizing logistical support for the al Qaeda cell that carried out the attacks. Western governments subsequently shut down a huge banking network in Switzerland, Liechtenstein and the Bahamas that was set up by a leading Brotherhood figure, citing its numerous financial ties to al Qaeda and other terrorists. The founder, Youssef Nada, denies wrongdoing.

In March 2002, federal agents in Northern Virginia raided a cluster of Muslim think tanks, companies and foundations run mostly by men who sympathized with the Brotherhood in Iraq and elsewhere in the 1960s. No charges have resulted, but U.S. officials stated in court earlier this year that they are pursuing terrorist financing allegations. Members of the group, known for their relative political moderation, say they ended Brotherhood ties years ago and deny wrongdoing.

In a 42-count indictment in July, the government alleged that an Islamic charity, the Texas-based Holy Land Foundation for Relief and Development, funneled $12.4 million to a designated terrorist group, Hamas. The indictment said the Holy Land Foundation was "deeply involved with a network of Muslim Brotherhood organizations dedicated to furthering the Islamic fundamentalist agenda espoused by Hamas." The Holy Land Foundation denies wrongdoing.

The indictment alleges that the Holy Land Foundation and its Brotherhood allies performed services for Hamas -- fundraising, banking, producing videos and distributing literature. The Brotherhood network, according to the indictment, also hosted conferences -- featuring Hamas officials and radical sheiks -- that glorified extremism and included "violent dramatic skits depicting the killing of Jewish people." But the Brotherhood was not charged with any crimes.

One alleged Brotherhood figure is Soliman S. Biheiri, a Northern Virginia finance company executive convicted last year of lying to obtain U.S. citizenship. Biheiri, who federal documents say invested money for years in this country for Hamas officials, is "the Muslim Brotherhood's financial toehold in the U.S.," federal prosecutor Steven Ward said last year in court.

For law enforcement, the Brotherhood remains a worrisome enigma.

"The complication is they are a political movement, an economic cadre and in some cases terrorist supporters," said Juan Zarate, chief of the Treasury Department's terrorist finance unit. "They operate business empires in the Western world, but their philosophy and ultimate objectives are radical Islamist goals that in many ways are antithetical to our interests. They have one foot in our world and one foot in a world hostile to us. How to decipher what is good, bad or suspect is a severe complication."

Until recently "there wasn't a recognition of the logistical and financial ties to terrorism through the Muslim Brotherhood," Zarate added.

A senior U.S. law enforcement official said the FBI studied the Brotherhood -- or the Ikhwan -- from afar for a decade, but "we are more actively aware of them now." One worrisome feature of the network, he said, is the secret bond among Brotherhood activists. "We are very interested in relations among people and entities," he said. "People know each other for 20 years and will do anything for them because they are all 'brothers.' "

The Brotherhood has been connected to many Islamic extremists worldwide. Two Egyptian Brotherhood members went on to found split-off terrorist groups: Ayman Zawahiri, now Osama bin Laden's deputy, and blind sheik Omar Abdel Rahman, who was convicted in 1995 of plotting to blow up New York landmarks.

One top movement leader is Nada, who was jailed in Egypt in the 1950s for Brotherhood activities. He later became wealthy selling construction materials in Saudi Arabia, where he was called the "cement king," and now lives in a sprawling Italian villa.

With "significant backing from the Muslim Brotherhood," Nada set up a complex global banking network in the 1980s, the Treasury Department said when it recently designated Nada and two other Brotherhood officials as terrorist financiers. U.S. and European officials say the network has funded al Qaeda, Hamas and Algeria's Armed Islamic Group -- assertions that Nada denies. Although the network was supposedly shut down, U.S. and European officials say they still find Nada moving funds under new corporate names.

One of Nada's key aides has been a Holocaust revisionist from Switzerland, Ahmed Huber -- one of many neo-Nazis who helped the Ikhwan set up its financial structure.

Muslim activists who know current and former Brotherhood sympathizers in this country say bitter opposition to Israel is a key part of Brotherhood beliefs. Law enforcement sources say hundreds of current and former Ikhwan supporters nationwide are under federal investigation for alleged financial support of Hamas and other Palestinian groups deemed terrorists by the U.S. government.

But some Brotherhood experts say most wings of the movement are moderate and no threat to the United States. Ahmad Sakr, who has known Brotherhood activists in his native Lebanon and in this country, said U.S. officials are "100 percent wrong" to treat the Ikhwan with suspicion.

"They're not military men or terrorists," said Sakr, a Muslim activist in California. "They're educated and contribute to the success of America. The Muslim Brothers want to practice Islam in their families and in themselves, to show Islam is for every human being. . . . I never saw humble people like them. They give everything for the honor of God."

The Saudi Connection

The Muslim Brotherhood was founded in 1928 by a 22-year-old schoolteacher named Hassan Banna in his house in the Egyptian city of Ismailiyya. Banna railed against the colonial powers' humiliation of Muslims, and preached that governments should be ruled by Islamic law, or sharia.

Members swore obedience to Banna, pledging iron discipline and secrecy. They were organized into tiers of membership, with some forming a covert military wing to confront the Cairo regime.

As the Ikhwan's following grew to half a million, Banna was assassinated by Egyptian officials in 1949. Five years later, after Brotherhood members fired shots at Egyptian leader Gamal Abdel Nasser, thousands of Ikhwanis were imprisoned, shattering the organization. Hundreds more were jailed in Syria and Iraq. Another Brotherhood leader, Sayyid Qutb, who advocated militant jihad against nonbelievers and revolution against impure Muslim states, was hanged by Egypt in 1966. Qutb's books would later provide the philosophical underpinning for jihadists such as bin Laden as well as many Islamists in this country.

Today, the Egyptian Ikhwan operates semi-openly, with some members serving in parliament. It eschews violence against the government of Hosni Mubarak, arguing that it can attain its goals by peaceful proselytizing, one wayward soul at a time. Still, Mubarak has banned the Brotherhood, labeling it a terrorist front, and jailed hundreds.

Egypt remains the Brotherhood's center of gravity, with Mohammed Akef, its "Supreme Guide" in that country, considered by many to be the group's de facto leader worldwide. Akef embodies the contradictions of the movement, with statements supporting democratic elections as well as violence against Americans in Iraq.

In the 1950s, Brotherhood activists -- reeling from their suppression in Egypt, Iraq and Syria -- found a refuge in Saudi Arabia, newly awash in oil money. Thousands of Ikhwanis became teachers, lawyers and engineers there, staffing government agencies, establishing Saudi universities and banks, and rewriting curricula.

With royal family approval, Brotherhood activists also launched the largest Saudi charities, including the Muslim World League in 1963 and the World Assembly of Muslim Youth in 1973. Funded by petro dollars, they became global missionaries spreading the Saudis' austere and rigid Wahhabi school of Islam, whose adherents at times describe all non-Wahhabis as infidels.

The missionary work morphed into armed struggle in Afghanistan, where in the 1980s Saudi-financed Brotherhood activists helped repel the Soviet invasion, with support from the CIA and Pakistan. As Islamic radicalism spread with the Soviet defeat in Afghanistan in 1989, many Ikhwanis laboring for the Saudis embraced worldwide jihad and were at al Qaeda's inception.

The Brotherhood began to fall out of favor with the Saudis in 1990, when the Ikhwan backed Iraqi leader Saddam Hussein in his invasion of Kuwait. The Saudis slowly cut off funding.

After the Sept. 11 attacks, Saudi leaders began describing the transnational Brotherhood as the germ of al Qaeda while playing down the role of its government-backed clergy. Recently, Saudi Interior Minister Prince Nayef repeatedly denounced the Brotherhood, saying it is guilty of "betrayal of pledges and ingratitude" and is "the source of all problems in the Islamic world."

Coming to America

In the 1960s, Brotherhood activists started arriving in the United States. Most embraced modernism and American culture, people who sympathize with them said. Many also ended a formal tie to the Cairo-based Ikhwan headquarters even as they hewed to Ikhwan principles. Among their main goals were carving out havens for Muslims, propagating Islam in America and backing Israel's destruction, said Ali Ahmed, a Saudi activist in Washington close to many Ikhwanis.

"In this country the Ikhwan is mostly not a formal membership organization but a set of ideas people subscribe to," Ahmed said. "A lot of Brotherhood people who came here became more moderate and interested in democracy, while others became more radical."

A U.S. official familiar with federal investigations of former Brotherhood members said some developed "a disciplined strategy, specific goals" to act on their plan to convert Americans, starting with U.S. military personnel, prison inmates and black people.

Many Brotherhood leaders advocate patience in promoting their goals. In a 1995 speech to an Islamic conference in Ohio, a top Brotherhood official, Youssef Qaradawi, said victory will come through dawah -- Islamic renewal and outreach -- according to a transcript provided by the Investigative Project, a Washington terrorism research group. "Conquest through dawah, that is what we hope for," said Qaradawi, an influential Qatari imam who pens some of the religious edicts justifying Hamas suicide bombings against Israeli civilians. "We will conquer Europe, we will conquer America, not through the sword but through dawah," said the imam, who has condemned the Sept. 11 attacks but is now barred from the United States.

In his speech, Qaradawi said the dawah would work through Islamic groups set up by Brotherhood supporters in this country. He praised supporters who were jailed by Arab governments in 1950s and then came to the United States to "fight the seculars and the Westernized" by founding this country's leading Islamic groups.

He named the Muslim Students Association (MSA), which was founded in 1963. Twenty years later, the MSA -- using $21 million raised in part from Qaradawi, banker Nada and the emir of Qatar -- opened a headquarters complex built on former farmland in suburban Indianapolis. With 150 chapters, the MSA is one of the nation's largest college groups.

The MSA Web site said the group's essential task "was always dawah." Nowadays, Muslim activists say, its members represent all schools of Islam and political leanings -- many are moderates, while others express anti-U.S. views or support violence against Israelis.

Some of the same Brotherhood people who started the MSA also launched the North American Islamic Trust (NAIT) in 1971. The trust is a financing arm that holds title to hundreds of U.S. mosques and manages bank accounts for Muslim groups using Islamic principles.

In 1981, some of the same people launched the Islamic Society of North America (ISNA), which was also cited in Qaradawi's speech. It is an umbrella organization for Islamic groups that holds annual conventions drawing more than 25,000 people. Some U.S. officials praise its moderation, and its Islamic Horizons magazine covers such topics as Muslim Boy Scouts and Islamic investing principles.

People who helped set up the MSA, NAIT, ISNA and related groups say they are in no way anti-American -- they say they embrace American values while trying to strengthen their Muslim identities. They say their goal is not converting all Americans to Islam but constructing a vibrant Muslim community here.

The MSA, NAIT and ISNA did not respond to requests for comment. Officials from those organizations have said elsewhere they are not connected to foreign groups, such as the Brotherhood. But because the Brotherhood is a secret society, its precise links around the world are hard to determine, U.S. officials said.

In addition to the first generation of groups aimed at consolidating the U.S. Islamic community, a second generation arose to wield political and business clout.

One such group was the American Muslim Council (AMC), launched in 1990 to urge Muslims to get involved in politics and other civic activities. One of its founders was Mahmoud Abu Saud, who 58 years before helped Banna expand the Brotherhood, and who later became a top financial adviser to governments from Morocco to Kuwait, according to documents provided by the SITE Institute, a Washington terrorism research group that has written reports critical of the Brotherhood. The AMC folded in 2003, and a more moderate group has assumed that name.

One leader of the former AMC was Abdurahman Alamoudi, who U.S. officials and Islamic activists say is a Brotherhood associate. In July he pleaded guilty to moving funds from Libya, which was illegal because the United States at the time considered that country a sponsor of terrorism. Federal documents in the case say he is a Hamas supporter. Alamoudi also was identified by U.S. officials in June as a participant in a plot hatched by Libyan leader Moammar Gaddafi to assassinate the Saudi head of state, Crown Prince Abdullah.

Another group in this generation is the Muslim American Society, based in Falls Church, which was co-founded in 1992 by Akef, the recently installed head of Egypt's Brotherhood, and other Ikhwanis, Akef told the Chicago Tribune in February. The group's goals include spreading Islam to Muslims and non-Muslims and building "a virtuous and moral society." Its officials deny ties to the Ikhwan.

Home in Northern Virginia

Since the mid-1990s, a Northern Virginia-based group of companies, charities and think tanks has also been under off-and-on scrutiny by U.S. officials looking into whether it has ties to anti-Israel terrorist financing. Lawyers for the informal network, centered on the Herndon-based International Institute of Islamic Thought (IIIT), deny impropriety.

Jordanian political figure Farhan, who was barred from this country in 2000 and then received U.S. diplomats' apologies in Amman, had been on his way to a Virginia board meeting of the IIIT, which he had helped lead for years.

Lawyer Nancy Luque said her clients embrace American values such as democracy and equality for women. "They love this country," she said. "Their kids are in school here becoming doctors and lawyers." In the 1980s and early 1990s, she said, her clients gave intelligence tips picked up by their global contacts to the State and Defense departments.

The IIIT network was set up in the 1980s largely by onetime Brotherhood sympathizers with money from wealthy Saudis, Muslim activists said. A number of its members ended their Brotherhood ties years ago after concluding it was too inflexible but still advocate some of its principles, the activists said.

Some network figures had dealings with activists who ran two vehemently anti-Israel groups out of the University of South Florida in Tampa, federal documents said. One of the activists, USF professor Sami al-Arian, was indicted last year on charges of conspiracy to commit murder via suicide attacks in Israel. Officials said he was secretly a top leader of the Palestinian Islamic Jihad terrorist organization. Al-Arian denies the charges.

The network's lawyers say that its ties to Al-Arian were fleeting. The government is looking into whether the network engaged in tax violations and "suspected terrorism-related money laundering activities," a U.S. customs agent stated in a report on the probe filed in federal court a year ago.

Luque said her clients abhor terrorism, including against Israelis.

But an IIIT book called "Violence," published in 2001, said Israel is a "foreign usurper" that must be confronted with "fear, terror and lack of security." The book, by IIIT official AbdulHamid AbuSulayman, says, "Fighting is a duty of the oppressed people." Palestinian fighters must choose their targets "whether the targets are civilian or military," it said, adding that any such attacks should not be "excessive." The book said such attacks are justified acts of a liberation struggle, not terrorism.

The life story of one of the IIIT network's leaders illustrates the key role it has played in the global politics of the Ikhwan. Jamal M. Barzinji fled his native Iraq in 1969 when the Baathist regime started executing fellow Islamists. An engineering student and top MSA leader, he joined MSA associates in 1971 to host the top leaders of the Egyptian Brotherhood, just released from 16 years in prison, for two weeks of meetings in Indiana.

He and other then-MSA leaders helped persuade the Egyptian brothers to try participating in Egyptian elections as an alternative to underground struggle, he said. "It was one of our main contributions to the Ikhwan movement worldwide," he said. He and his associates likewise have hosted many other Islamist leaders here over the years to "show them how wrong they are in being anti-American," Barzinji said.

But the government's current probe of the IIIT network undercuts their efforts toward moderation, he said. "The extremists say: 'See? All American society is corrupt.' "

Beyond U.S. Shores

Some U.S. Islam experts say law enforcement investigations of Ikhwan-tied activists complicate U.S. diplomatic dialogues with Brotherhood members overseas. For years, State Department and CIA officials have met with Brotherhood activists in Egypt, Kuwait, Jordan, the Palestinian territories and elsewhere to track currents within Islamic politics.

"We want to know where they're coming from, to influence them," said Edward P. Djerejian, a former top State Department official who now runs Rice University's James A. Baker III Institute for Public Policy.

At the same time, host governments in Cairo, Tunis and elsewhere warn that the Brotherhood is dangerous. So do many in U.S. law enforcement. "There were debates all the time about meeting with them," Djerejian said.

Since the Sept. 11 attacks, pockets in the government -- including officials in State's Near East bureau and diplomats posted overseas -- have quietly advocated that the government reach out to the Brotherhood and its allies. These officials and some in U.S. think tanks hope the Brotherhood can temper its anti-U.S. stance and become a barrier against jihadists worldwide.

"Bin Laden-ism can only be gutted by fundamentalists" such as the Ikhwan, said Reuel Gerecht, a former CIA officer in the Middle East who is tracking pro-democracy activism in the region for the conservative American Enterprise Institute.

As U.S. officials try to promote democracy in Muslim countries, he said, "it's inevitable the U.S. will engage the fundamentalists" because of their popularity in those societies. Indeed, many Arab experts say the Ikhwan or its allies could win open elections in countries such as Egypt and Algeria.

But many in the government oppose engagement because it runs counter to the wishes of close U.S. allies in the Egyptian and Moroccan governments, which feel threatened by the Brotherhood.

"At high levels of the government, there's no desire to go in the direction of dialogue," said Fuller, the former CIA official. "It's still seen as fairly way out." But he warns against a litmus test for talking to Islamists -- such as eliminating those who embrace anti-Israel terrorism or make anti-American statements. "There's hardly an Islamic group anywhere that hasn't done that," he said.

Leslie Campbell, who runs Middle East affairs for the National Democratic Institute for International Affairs, supports the outreach idea. Campbell, who trains Arab politicians including Islamists, hosted a delegation in July from a Brotherhood-tied political party in Yemen to the Democratic National Convention in Boston.

"They appreciated that the U.S. had reached out to them," he said. "If they're empowered, they'd serve as a bulwark against those who want to destroy."

Research editor Margot Williams contributed to this report.

Anti-terrorism investigations included this 2002 raid on offices in Herndon. The International Institute of Islamic Thought was tied to groups in the case.The Egyptian Ikhwan is opposed to violence against the nation's government but is still considered a terrorist front.