The four men charged with plotting to blow up two New York synagogues and shoot down military planes converted to Islam while behind bars, a place terror experts say is a cauldron for Islamic radicalism.

James Cromitie, David Williams, Onta Williams and Laguerre Payen were said by friends and family to have become Muslims while in prison, where they began on a path toward terrorist violence that law enforcement officials say is part of a troubling trend in America.

"Prisons can play a critical role in both triggering and reinforcing the radicalization process," read a landmark 2007 report from the New York Police Department, which warned of prisons as a "radicalizing cauldron."

"The prison's isolated environment, ability to create a 'captive audience' atmosphere, its absence of day-to-day distractions, and its large population of disaffected young men, makes it an excellent breeding ground for radicalization."

Many inmates convert to Islam while incarcerated, but security experts have signaled an emerging threat from those who embrace "Prison Islam," which they say is a twisted version of the religion.

Prison Islam is "the convergence of prison culture and violence into religious practice," said Frank Cilluffo, director of the Homeland Security Policy Institute at George Washington University. Often lacking an imam or other religious leader to instruct them, inmates will distort Islam for themselves to embrace prison values like violence and gang loyalty.

And while religious conversion is rehabilitative for the vast majority of inmates, Cilluffo said, Prison Islam can lead them down a much more dangerous path. Charismatic leaders can recruit prisoners to their cause and further swell the ranks of the radicals.

Between 6 and 10 percent of the U.S. prison population practices some form of Islam, according to varying studies. Some join in good faith, some join for protection from gangs, some join for the Halal food they prefer to regular prison fare. It's the ones who become radicalized who pose a distinct threat, federal officials say.

"These radicalized inmates either feel discriminated against in the United States or feel that the United States oppresses minorities and Muslims overseas," Donald Van Duyn, now chief intelligence officer for the FBI, said in testimony before the Senate Committee on Homeland Security in 2006.

That seems to be the story of James Cromitie, the apparent ringleader of the four alleged terrorists from Newburgh, N.Y. According to the federal complaint filed against them, Cromitie told an FBI informant that "he was upset about the war" in Afghanistan and "unhappy that many Muslim people were being killed in Afghanistan and Pakistan by the United States Military forces."

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Cromitie spent time in prison from 2000-2004 and did an earlier stint in Fishkill Correctional Facility in Beacon, N.Y., where the Muslim chaplain was Salahudin Muhammad. Muhammad, who Newburgh city officials say preaches peace from his pulpit, is also the imam of that town's sole mosque, Masjid al-Ikhlas, which Cromitie is known to have attended before his arrest Wednesday night.

Muhammad, when interviewed by FOXNews.com, said emphatically that Muslims are not being radicalized inside the prison, called it a "sensational" charge meant to sell books.

"Nobody is teaching radicalism in the prisons. People are teaching prisoners how to get rid of their criminal mentality. That's what's being taught," he said.

While the FBI has identified some prison chaplains as sources of radicalization, some prison experts say the problem is that there are too few imams in the system, which must then contend with the inmates offering their own malformed teachings on the religion.

A Department of Justice report in 2004 found that there was an average of one Muslim chaplain for every 900 Muslim inmates in federal prisons, which it said constituted a "critical shortage." There are 40 imams serving the New York Department of Corrections, which runs 68 prisons.

"When you've got groups that go underground, that didn't rely on chaplains, that's when you've got some real trouble," Cilluffo told FOXNews.com.

One such group of ex-cons was preparing terrorist attacks on Jewish and government targets around Los Angeles in 2005 when it was disrupted by local law enforcement. A coordinated yearlong investigation brought down the cell allegedly planning attacks on the synagogues in New York this week.

Prison radicals have been most active on the West Coast and in the Northeast, and security experts say they don't currently pose a threat that is strong in numbers.

Cilluffo noted that conversion is a positive experience for the vast majority who take up the religion.

But it doesn't take large numbers to pose a substantial threat to America, he said.

"The reality is you don't need many people. One is arguably too much."