One balmy January evening just after the end of Ramadan, a Saudi graduate student named Omar Al-Bayoumi hosted a party to welcome a pair of recent arrivals into San Diego's large Muslim community.

The guests -- two dozen men to whom Al-Bayoumi had spread invitations at mosques around the city -- squeezed into the newcomers' first-floor garden apartment. They feasted on a whole baked lamb, a delicacy provided by their host, who circulated with a video camera.

And who were the polite young Saudis who'd moved to town? Nawaf Alhazmi and Khalid Almihdhar, two of the hijackers who would commandeer a commercial airliner and crash it into the Pentagon.

The party, held in the opening weeks of the new millennium, may be the most vivid of many moments during the past two years in which the Muslims of San Diego -- apparently unwittingly, with the possible exception of the party's host -- embraced two of the conspirators in the worst act of terrorism ever committed in the United States.

From their arrival here in late 1999 until they departed a few months before the Sept. 11 attacks, Alhazmi and Almihdhar repeatedly enlisted help from San Diego's mosques and established members of its Islamic community. The terrorists leaned on them to find housing, open a bank account, obtain car insurance -- even, at one point, get a job.

In this way, San Diego is distinct from most other places in the country where the 19 hijackers prepared for the attacks. In Florida, New Jersey and suburban Washington, the terrorists operated on the margins, relying on cheap hotels and rented cars. Here, they burrowed in; Alhazmi was listed in the San Diego phone book.

Because the two hijackers forged such visible ties while they lived here, the southern edge of California has emerged as a focal point of the federal government's massive terrorism investigation. Six Middle Eastern men who have lived here have been detained, including college students and an engineer for the state's transportation agency. A seventh, Al-Bayoumi, 44, the party host, has been held by authorities in England, where he moved this year.

In addition to these detentions, FBI agents have conducted "thousands of interviews" throughout the city, said William D. Gore, special agent in charge of the bureau's San Diego office. According to people who have been interviewed, investigators have scoured college campuses, shown up at Muslims' homes and workplaces and searched the financial records of at least one mosque the hijackers attended. This month, 10 young adults from Middle Eastern countries were arrested here on alleged student visa violations, becoming the first to be charged in a new, nationwide crackdown.

The intensity of the investigation here makes San Diego a useful vantage point from which to glimpse the role and treatment of suspects who have been ensnared in the government's campaign of detentions, a campaign that has put at least 1,200 people in custody nationwide as the Bush administration tries to prevent further acts of terrorism.

So far, law enforcement officials have been reluctant to discuss what they are learning. Interviews with dozens of community members suggest that most of the men caught up in the probe in San Diego shared housing, work or prayer with one or both of the hijackers, but little else. It is from these connections that investigators have tried to fathom any deeper relationship between the community and the terrorists.

Regardless of whether such a relationship is found, the investigation has affected people in the community beyond those who have been detained. The FBI and local police say they are eager to cooperate with the Middle Eastern population, a diverse and largely suburban group that has grown to more than 100,000 over the past decade as waves of Somalis, Kurds and Christian Iraqis have settled alongside Afghans, Algerians, Iranians, Saudis and other earlier immigrants.

Nevertheless, Muslims say the climate since Sept. 11 has been hostile. Officials and faculty at local colleges say most students from Middle Eastern countries have dropped out and moved home. Attendance at mosques has dipped. Many devout Muslim women have stopped wearing head scarves, fearful of being conspicuous, and Christian Chaldeans who moved here from Iraq have begun to wear large crosses in public, to distinguish themselves from Muslims.

Such caution does not appear unwarranted. The San Diego police department lists 55 incidents so far that it considers retaliation for the Sept. 11 attacks. The Islamic Center of San Diego, the largest of the city's 15 mosques, was struck twice by paintballs. A few days later, someone tossed a small bomb wrapped with BBs out front, but it did not explode.

There is a paradox to the anger directed at San Diego's Muslims and to Alhazmi and Almihdhar's decision to mingle here: This Islamic community is relatively devoid of radicalism, of anti-Western feeling. "The rhetoric, the individuals talking about jihad . . . I think it is relatively low," said the FBI's Gore.

In fact, investigators believe that the community's very openness, its relaxed and diverse nature, made this city of sunshine and beaches hospitable terrain for two men in their mid-twenties with poor English and a lethal plan.

'They Shocked Me'

If Nawaf Alhazmi and Khalid Almihdhar betrayed no hint of their intentions, they also made no effort in San Diego to hide their presence.

Their first home here, the dwelling where the welcome party took place, was a carpeted, one-bedroom unit in the Parkwood Apartments, a complex in the city's Clairemont section that teems with students and other young Muslims. It is blocks from the Islamic Center.

It was at the mosque that the pair noticed an ad posted by a retired educator from India who had decided to rent out rooms after he was divorced and his children were grown. In September 2000, the two young Saudis began paying $300 a month to share a bedroom in the house of Abdussattar Shaikh, a courtly man of 65 known as "the doctor" around Lemon Grove, a suburb east of the city in which he has lived for more than 20 years.

"They shocked me," Shaikh said, recalling the September day he heard on the radio that his former tenants had taken part in the attack on the Pentagon. "If I had any hint, I would have tipped off the authorities."

In the three months they lived with him, Shaikh said, they told him they were students who had come to learn English. But Alhazmi spoke little English, Almihdhar spoke none, and they seemed disinclined to practice, even when encouraged by Shaikh, who had run Montessori schools and academies for teaching English to foreigners before he retired. Because Shaikh is not fluent in Arabic, communications with the two Saudis were clumsy.

Their landlord was unaware that they took a few flight lessons and were asked to leave because of their lack of ability. They told him they were Bedouin, Arab people of the desert. Shaikh said they never ate at the kitchen table, but on the kitchen floor. The young men mainly hung around the house. They prayed. They did not watch television, read books or make phone calls from home. Nor did they speak of Middle Eastern politics.

But Alhazmi, in particular, asked Shaikh for help -- in composing a personal ad on the Internet for a Mexican bride who might be willing to convert to Islam, in opening an account to deposit $3,000 at the Bank of America's Lemon Grove branch, in shopping for used tires for a car he'd just bought. It was the Toyota Corolla that the hijackers of American Airlines Flight 77 would leave in a parking lot at Washington Dulles International Airport the morning they died.

An accommodating landlord was not the only thing the two hijackers found through the city's mosques.

From another mosque, Masjid Ar-Ribat Al-Islami, Alhazmi got a recommendation for a job at a nearby Texaco station and car wash, a business whose former owner, a Muslim, was known to help young men in need of work. For a month, Alhazmi worked at minimum wage two days a week, vacuuming and drying cars.

"He was super nice. . . . He was always on time, which is important with me," said the manager at the time, who asked to be identified only by his first name, Ed. "He was a quiet guy, nice with customers."

And at the Islamic Center, the pair found an ad on a bulletin board that led them to the blue 1988 Corolla, which was being sold by Azzedine Abbadi, who lives a few blocks away. "He doesn't know them. He just sold a car," Abbadi's wife said.

After they bought the car, the hijackers yet again found what they needed. On Feb. 28, 2000, Almihdhar arrived at a branch of the Huggy Bear Insurance Agency, which advertises in the Yellow Pages that it welcomes customers with international driver's licenses.

When Almihdhar was asked for an address, he did not list his own, according to records provided by Ben Huggins, the company's owner. He gave Abbadi's.

A Social Host

Among the people who helped the San Diego hijackers, the one who seems to have done the most is Al-Bayoumi, the business student who orchestrated the welcoming party.

Exactly how Al-Bayoumi knew Alhazmi and Almihdhar remains obscure, but one San Diego source with strong ties to local Muslims said that Al-Bayoumi drove to Los Angeles late in 1999 to meet the two young men and "brought them down physically in his car."

The San Diego Union-Tribune has reported that Al-Bayoumi found and paid the rent for Alhazmi and Almihdhar's unit at the Parkwood Apartments, where he also lived at the time. A manager for the complex declined to comment. A spokeswoman for Parkwood did not return telephone messages.

Al-Bayoumi struck people as highly social. He attended many of the city's mosques and made a wide network of acquaintances. He identified himself as a graduate student, but people were unclear about what he was studying.

He somehow knew an affluent man in Saudi Arabia who in 1997 gave $545,000 to a group of poor Kurds in San Diego who wanted a larger building for their mosque. The benefactor gave the gift on the condition that Al-Bayoumi become the new building's maintenance manager, according to several people familiar with the transaction.

"He was a representative of the [donor]," said A. Halim Mostafa, a Kurd who has lived in San Diego for 23 years, owns a security business and was suspicious of the gift. "Why suddenly this money came, and this [Saudi] gentleman [Al-Bayoumi] entered the non-Arab [Kurdish] community?"

Earlier this year, Al-Bayoumi moved to Birmingham, England, with his wife and four children.

On Sept. 21, British authorities arrived at his door. After taking Al-Bayoumi to a West London police station, investigators took control of his house and another one where he had lived in recent months, next door to a Birmingham mosque. They ripped up floorboards, excavated the gardens and dismantled a picket fence to dig beneath the posts. Neighbors watched as police carried away plastic bags filled with unspecified evidence and hauled away his aging silver BMW.

But Al-Bayoumi was released after a week, the maximum permitted under Britain's anti-terrorism law. He remains in Birmingham today. Soft-spoken, wearing a wool sweater and jeans, he was working on a research paper earlier this month in a tidy 12th-floor office at Aston University's business school.

He declined to discuss whether he paid for the hijackers' first San Diego apartment or why he gave them a party. "I can't talk about the situation," he said in flawless English. "If you need anything, call my embassy. I don't have anything to hide."

Asked whether his difficulties with the British authorities were through, he replied: "I'm not sure. I haven't any clue."

It is unclear whether Al-Bayoumi has been rearrested. A week ago, British authorities detained eight foreigners under a strict, new anti-terrorism law. Sources said the arrests included two people in Birmingham, but several law enforcement and immigration agencies refused to confirm whether Al-Bayoumi was among them.

In any case, Al-Bayoumi's material and legal circumstances for most of the past few months appear far more comfortable than those of the men who have been detained in the United States as part of the San Diego investigation.

Some Detainees Charged

For the most part, these other detainees appear to have been connected to Alhazmi and Almihdhar inadvertently, by a web of overlapping addresses and work. Like detainees elsewhere in the country, they have been charged with immigration violations and, in a few instances, criminal offenses unrelated to terrorism. But there is no clear correlation between their proximity to the hijackers and how long they've been held.

The engineer for the California Department of Transportation, Zineddine Tirouda, 37, lived in the same apartment complex as Al-Bayoumi but never met him, according to Tirouda's lawyer, Kenneth Luis Chapman. Omar Bakarbashat, 28, a college student from Yemen, worked at the same Texaco station as Alhazmi and -- months after the hijackers left -- rented a room in Shaikh's house. Another Yemeni student, Ramez Noaman, 27, also roomed with Shaikh after the terrorists did.

Now, Tirouda has been charged with conspiring to obtain a false U.S. passport. U.S. officials have charged Bakarbashat with overstaying his student visa, misusing immigration documents and Social Security fraud. Noaman was arrested near Los Angeles and held in New York for two weeks as a material witness. All three were unavailable for interviews; attorneys said they had no role in the plot.

The other detainees, however, had slightly closer interactions with the hijackers.

Yazeed Al-Salmi, 23, is a Saudi student who also briefly lived with Shaikh -- after the two hijackers were there. And he shared a car insurance policy with Alhazmi so they could save money, according to Randall Hamud, one of his lawyers.

According to Hamud, Al-Salmi had wanted to follow the example of his father, who attended college in the United States. He is studying accounting at Grossmont Community College on a student visa valid through next summer, Hamud said. After being arrested in late September at a local mosque, he was held as a material witness for two weeks, testified before a grand jury in New York, and was released.

Two of Al-Salmi's former roommates also knew the hijackers; in at least one case, fairly well.

Mohdar Abdallah graduated from Grossmont and is a junior at San Diego State University. He was an assistant manager at the Texaco station where Alhazmi briefly worked and was introduced to the hijackers "as someone who was very quick with English," said Hamud, who has also represented Abdallah. "That was his downfall," because the hijackers found his language skills useful. "He saw them more than a few times."

For the last three months, Abdallah has been held on an immigration charge, accused of lying on an application for political asylum by claiming to be a Somalian refugee even though he is from Yemen and had been living in Canada before he moved to the United States.

A former professor at Grossmont said Abdallah was running out of money and could not afford to stay in school unless he held a job, which his student visa did not permit. "He was struggling to establish himself legally. . . . In desperation, he ended up applying for political asylum," said the sociology professor, Bachir Idiou, who advises the campus Muslim Student Association.

The detention of another Grossmont student, a former roommate and co-worker of Abdallah's named Osama Awadallah, illustrates the lengths to which federal investigators are going to try to keep young Middle Eastern men in custody.

Awadallah, 21, a Jordanian with a green card from the Immigration and Naturalization Service, also worked at the Texaco station, according to his older brother, Jamal Awadallah, a San Diego salesman who has become an American citizen. Their acquaintance was "very casual and very brief," his brother says. Yet investigators were intrigued that the name "Osama" and an old phone number for him were scribbled on a map inside the Toyota left at Dulles on Sept. 11.

Brought before the New York grand jury, Osama Awadallah, handcuffed to a witness chair, testified that he knew Alhazmi but said he did not remember Almihdhar, according to court documents. Prosecutors then showed him a journal he had kept for an English class. In the blue booklet filled with his small handwriting, he had mentioned that he knew both of them. He later told the grand jury his initial testimony had been confused; he has been charged with lying before the grand jury.

The teacher for whom he kept the journal thinks it is inconceivable he was part of a terrorist plot. Her student was young, naive and "very in-your-face" in his Islamic fervor, recalled the teacher, who asked not to be named. But he went to her distraught, after the attacks, telling her he was worried they would give his religion "a black eye."

"Was he a fanatical Muslim? Yes. Did he know the two hijackers? Yes," the teacher said. "But did he come to the United States for criminal or terrorist purposes? I don't think so."

Awadallah was held in solitary confinement for nearly three months at New York's Metropolitan Correctional Center. According to court documents filed by one of his lawyers, Jesse Berman, a guard threw shoes at Awadallah's head while he was held temporarily in an Oklahoma City jail, and a guard in New York pushed him into a wall and pulled his hair, forcing him to face an American flag.

During the past few weeks, federal judges in New York and in San Diego have set bond for Awadallah and Abdallah at $500,000 apiece. Awadallah was released on Dec. 13. Abdallah remains in custody, unable to post bond.

Hamud, the lawyer, now finds himself as Abdallah's fundraiser. It is, he said, difficult work. Many in the Muslim community, he said, are reluctant to give money, fearing such generosity might lead the FBI to their doors. Some donated at first, Hamud said, but changed their minds when they learned that their names and the source of their money would be reported to the court.

First Terrorists, Then FBI

Along with the hidden head scarves and reduced mosque attendance, the sluggish fund drive is one of many signs that the Muslims of San Diego are unsettled now -- first by the discovery that a pair of terrorists had lived in their midst, then by the FBI's interest in their community.

At the Texaco station, Ed, the manager, and his former boss telephoned the FBI, deciding it was better to approach the bureau first.

Shaikh, the landlord and retired educator, has been visited repeatedly by investigators. In his case, the FBI has deviated from its typical secrecy to issue a statement that he is not a suspect.

Still, a vandal ripped the copper nameplate off his front door. One day, a group of young men he did not know arrived "with knives to kill me," Shaikh said. An acquaintance, who happened to be visiting at the time, recalled that he told the men to leave and was thrown to the ground.

Hamud, the lawyer, is one of few people in the local Islamic community who have denounced the investigation publicly, contending that it is a form of racial targeting and a violation of civil liberties. A telephone caller said Hamud was a murderer. Another said a bomb would be delivered to his home or his office downtown.

Like many local Muslims, Omar Abdeen, a physician and leader at the Islamic Center, is ambivalent. He believes investigators need help in untangling the hijackers' plot, and he has been touched by offers of help from local religious and political leaders. But he still worries that investigators have singled out Muslims unfairly. "Why are we targeted?" he asked. "Just because we share the same faith as terrorists claimed to adhere to?"

"Many Muslims come from countries where being interviewed by law enforcement is not a good thing," Abdeen said. "That kind of fear stays."

Staff writer Joe Stephens and researcher Margot Williams contributed to this report. Stephens reported from Birmingham, England.

Osama Awadallah, left, with attorney Jesse Berman. Awadallah testified that he knew hijacker Nawaf Alhazmi.Yazeed Al-Salmi, center, is escorted by friends after being released by FBI as material witness. He shared an auto insurance policy with Alhazmi.