The arrest of four Muslim ex-convicts in an alleged homegrown terror plot in the Bronx is renewing fears about the spread of Islamic extremism in the nation's prisons. At least two of the four men suspected of plotting to bomb synagogues and shoot down military airplanes converted to Islam behind bars. The alleged mastermind is also a convert, and the fourth man identified himself as a Muslim when he entered prison. Islam has had a strong presence in US prisons for decades, and many chaplains and corrections officials credit the faith, when taught properly, with being a stabilizing force that can help inmates turn their lives around. But this week's foiled plot is not the first terror scheme implicating Muslim convicts, and it comes despite reports of progress in screening chaplains and materials on Islam in the prison system. "Basically, the threat is real," said Paul Rogers, past president of the American Correctional Chaplains Association. "Prisons have unstable people and people who are on the edge of a lot of different things. The radical elements of any religion can be emphasized." Those fears were heightened this week as lawmakers debated the fate of detainees if President Barack Obama shutters the prison at Guantanamo Bay. FBI Director Robert Mueller said terror suspects brought to the US could end up "radicalizing others" or plan attacks on the country. Defense Secretary Robert Gates said Obama would do nothing to endanger the public and decried "fear-mongering about this." The four defendants in the New York terror case had been in and out of prison. Laguerre Payen said he converted to Islam in prison, but a Muslim prayer leader who counseled him when he got out said he had a poor understanding of the faith. Onta Williams had registered as a Baptist in prison, but his uncle said he converted to Islam inside. David Williams and James Cromitie had registered as Muslim in prison, according to correction officials. Payen appears to be a Haitian citizen, while the three others are Americans. The Williamses are not related. Mitch Silber, a top New York Police Department intelligence analyst, said inmates converting to Islam are so common that he and his colleagues call it "Prislam." Though many drop the faith once they are out, for some "the conversion sticks" and can fuel anger toward the United States, said Silber, co-author of the 2007 NYPD report "Radicalization in the West: The Homegrown Threat." Just as young people can be radicalized by "cut-and-paste" readings of the Quran on the Internet, new inmates may get a distorted view of Islam from gang leaders or other influential inmates, according to "Out of the Shadows: Getting Ahead of Prison Radicalization," a 2006 report by the George Washington University Homeland Security Policy Institute and the University of Virginia Critical Incident Analysis Group. Several imams used the term "Jailhouse Islam" to describe a form of Islam in prison that incorporates gang loyalty and violence, the report said. Many states are doing a better job of screening the reading material that comes into prisons, Rogers said. But other problems arise when there are no qualified chaplains or volunteers. "Sometimes inmates rely on other inmates, and it's sort of the prison way," Rogers said. "They turn to someone they trust, their 'celly' or someone in their cellblock, and put them on a pedestal as someone who has more knowledge about the religion. He could be spreading knowledge, or could be spreading ignorance." Inmates who were radicalized in prison include: - Jose Padilla, a former Chicago gang member, who converted to Islam behind bars and was recruited at a mosque to become a mujahedeen fighter, authorities said. Prosecutors accused him of plotting to detonate a radioactive "dirty bomb," but he was convicted of unrelated terror support charges. - Richard Reid, a British citizen and follower of Osama bin Laden, who was a prison convert in England and became involved with militants after he was freed. He pleaded guilty in 2002 to trying to blow up a trans-Atlantic flight with explosives hidden in his shoes. He is now serving a life sentence at a maximum-security prison in Florence, Colo. - A group of prisoners in California who converted to Islam and were arrested in 2005 on charges of plotting behind bars to attack military sites, synagogues and other targets. Islam took hold in U.S. prisons in the 1940s, when members of the Nation of Islam were held for refusing to fight in World War II. Malcolm X was one of their most famous prison recruits. The religion spread when Muslim inmates successfully sued for more freedom to practice their faith. Get-tough sentencing laws enacted in the 1980s filled prisons with large numbers of blacks and led to another spike in conversion. By this time, many blacks who once were in the Nation of Islam embraced mainstream Islam instead. Some inmates gravitate toward Islam out of resentment over being locked up, said Jimmy Jones, a Muslim chaplain and volunteer in Connecticut prisons. "The argument being made is that some people are being taught core values in jail that are turning them into jihadists. This is outrageous," Jones said. "It's not Islam that turns them against America. If you track them back, they had some resentment already going on." Jones said Connecticut and New York correction officials have barred "The Noble Quran," a Saudi translation of the Muslim holy book. An appendix in the book is titled "The Call to Jihad (Holy Fighting in Allah's Cause)." But Jones argued that the threat of extremism from prison conversions has been exaggerated. "I think this is another form of fear-mongering," he said. "The guys I know who come out of prison, they have many issues, and that's not one of them." Harry Dammer, a criminologist at the University of Scranton in Pennsylvania who studies religion in prisons, said there is no clear evidence that the Islam taught or spread in U.S. prisons are the forms of Islamic militancy on display in the Bronx case. "I would say that yes, of course, there are extremists around, but they are few and far between," Dammer said, "and they are not supported by imams and chaplains in prisons."