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 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 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."