It's Friday on Rikers Island, time for weekly worship for almost a quarter of the city jail's 14,000 inmates.

The men, Muslims, file quietly into a classroom of white cinderblock that serves as their mosque and sit on sheets stamped "Department of Corrections" covering the linoleum floor. Imam Menelik Muhammad stands before a wall facing Mecca and preaches.

"You will not be considered a Muslim unless people are considered safe from your hands and your tongue," he tells the prisoners.

Across the United States, tens of thousands of Muslims are practicing their faith behind bars. Islam is most likely to win American converts there, according to U.S. Muslim leaders, and the religion has for decades been a regular part of prison culture.

But the Sept. 11, 2001, attacks have brought new scrutiny to Muslim inmates, many of whom are black Americans. Prison chaplains of various faiths say Islam offers a path to rehabilitation, but others say it has the potential to turn felons into terrorists.

FBI Director Robert S. Mueller III told the Senate Intelligence Committee in February that "prisons continue to be fertile ground for extremists who exploit both a prisoner's conversion to Islam while still in prison, as well as their socioeconomic status and placement in the community upon their release."

The reality is harder to read: Those on opposing sides have such divergent views they seem irreconcilable. Who's right matters not only for national security, but also for the development of American Islam itself.

Ever since the 2002 arrest of Jose Padilla, an American Muslim convert who authorities say planned a "dirty bomb" attack after he left jail, politicians, law enforcement officials and even a few evangelical leaders have warned that Muslim inmates are ripe for terrorist recruitment.

Chuck Colson, founder of Prison Fellowship Ministries and a Nixon administration official, predicted that "radical Islamists will use prisons" to avenge Islam.

Prison chaplains and others, however, say such warnings are dangerously ignorant.

In interviews with the Associated Press, chaplains, prison volunteers, correctional officials, inmates and former inmates all insisted that there was no evidence of terrorist recruitment by Muslims in their prisons -- although banned pamphlets and books sometimes slip in.

Chaplains describe the typical inmate convert as a poor, black man upset about racism, not Mideast politics; someone who turned to Islam to cope with imprisonment. When they get out, these men are so overwhelmed by alcoholism or poverty that the crimes they are most likely to commit are the ones that landed them in jail to begin with, chaplains say.

"They don't care about Osama bin Laden," said imam Talib Abdur Rashid, who worked for years as a prison chaplain in New York state. "They have their own beefs that have nothing to do with [Islamic law], the Taliban or Wahhabism, and everything to do with slavery, segregation and the history of U.S. racism."

If extreme teachings are reaching U.S. prisoners, experts say small-time operators acting alone are more likely to be responsible than an underground movement or a professional chaplain.

Still, radicals have historically been successful with recruiting behind bars, and there is growing concern about militants looking for inmate recruits in other countries. That is fueling fear about American prisons, where defining the Islamic presence among inmates is tricky.

Though they make up about 6 percent of roughly 150,000 federal inmates, there are no nationwide statistics on Muslims in state prisons. Experts believe the largest populations can be found in states such as New York, where Muslims are roughly 18 percent of the 63,700 inmates, and Pennsylvania, where the figure is about 18 percent out of 41,100.

Overall, the percentage of American Muslims in prison is considered higher than it is in the general population, where the number of Muslims could be as high as six million, or roughly 2 percent.

Islam took hold in prison in the 1940s and spread for decades through the Nation of Islam, which mixes black nationalism with Muslim traditions. But by the 1980s, most inmates embraced orthodox Islam instead -- and that is what the majority practice today.

Some inmates become Muslim to seek protection from gangs, enjoy privileges like holiday meals or escape the monotony of prison life. But for others, the change is authentic, and correctional officials say Islamic observance helps maintain prison security.

Duval Rafq, who was convicted of rape and became Muslim two years into his Connecticut prison sentence, said converting led him to accept responsibility for his crime.

"My behavior all of a sudden changed and other people's attitude and behavior toward me changed," said Rafq, who was released five years ago and now works while attending night school.

Despite such success stories, some lawmakers and analysts remain convinced that radicals still have access to inmates -- and they note that just one militant inmate could create enormous risk.

The Institute of Islamic Information and Education, based in Chicago, was one example cited at a 2003 Senate Judiciary Committee hearing on terrorist recruitment in prisons and the military.

The traditionalist institute sends books on Islam to prison chaplains and says it responds to more than 3,000 letters a year from inmates inquiring about Muslim teachings.

But its founder, Amir Ali, also runs another Web site.

In those postings, he calls al Qaeda leader bin Laden a "true Muslim" who wouldn't hurt anyone, says Jews control the media and contends Hollywood producers fabricated videotapes of bin Laden threatening more violence.

Yet it's unclear what danger his opinions pose because Ali's reach to prisoners from the Web site cannot be measured.

Inmates are barred from using the Internet and all materials sent to prisons are vetted by chaplains and correctional officials, though officials concede that banned publications sometimes get through. Several Christian, Jewish and Muslim chaplains around the country said in interviews that they had never heard of Ali or his institute.

Ali, in a phone interview, said he keeps his political views separate from his religious outreach, which at one time was partially funded by a Saudi Arabian organization.

"As a citizen of this country, I believe I have a right to my views," said Ali, who emigrated from Pakistan and said he's never been contacted by the FBI. "There's nothing secret about it. None of this material goes to prison and none of this material goes to anybody except those who visit this Web site."

Another pressing issue for law enforcement is the background of chaplains working inside prisons.

Since Islam has no central authority or the equivalent of a major seminary in the United States, most prison officials turn to local Muslim leaders for guidance, said Paul Rogers, president of the interfaith American Correctional Chaplains Association.

In New York state prisons, which are separate from city-run Rikers Island, some say those safeguards failed.

Imam Warith Deen Umar, who retired in 2000 after two decades leading New York state Muslim chaplains, seemed to express support for the Sept. 11 attacks in a 2003 interview with the Wall Street Journal.

Umar, who said he was misquoted and did not support the hijackings, was subsequently banned from New York prisons and lost a chaplaincy contract in federal prison.

Other chaplains said Umar, who spent two years in prison as a young man for a weapons possession crime, should not be seen as typical of prison imams, since ex-convicts have been barred from the chaplaincy for about two decades.

If the government has found new evidence of Islamic militancy in prisons, it's hard to know because of the secrecy surrounding terrorist investigations.

An FBI spokesman would not comment, though an agent told a conference for correctional officers in September 2003 that there have been no documented cases of convicts joining al Qaeda in prison. Ten federal Muslim chaplains told Justice Department investigators in a report last year that they had seen no attempts by terror groups to radicalize inmates.

A related, and some say bigger, challenge for law enforcement is monitoring inmates when they get out. Padilla, who has not been charged, turned radical after he was released, federal agents said, as did Richard Reid, a British convert who tried to blow up a transatlantic flight with explosives in his shoe.

Chaplains say a culture change in U.S. prisons makes the spread of extremism less likely.

Back in the 1970s, many Muslim inmates were veterans of black nationalist movements who felt a link to Third World anti-colonial struggles and antipathy toward U.S. government policies.

Jimmy Jones, a Muslim who worked for about 25 years as a chaplain in a New Haven, Conn., jail, said that way of thinking is no longer the norm. Jones said he heard a couple of young inmates cheer the Sept. 11 attacks, but he contended their response came from "adolescent bitterness" about being incarcerated.

"I think people are confusing what people say with what people might do. The younger inmates don't know anything about the Third World or about Egypt or the Middle East," said Jones, a world religion professor at Manhattanville College. "Al Qaeda would have more success recruiting at a college than in prison."

A Rikers Island inmate faces Mecca to pray, above, and Imam Menelik Muhammad, left, waits for inmates to arrive for prayers. Prisoners have been finding Islam for decades, but since 9/11, some fear terrorism may be winning converts, too.Imam Menelik Muhammad begins Friday afternoon worship at Rikers Island.