NEW YORK — The arrest of four Muslim ex-convicts in an alleged homegrown terrorist 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 U.S. 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 terrorism scheme implicating Muslim convicts.

“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 terrorism suspects brought to the U.S. 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 terrorism 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.