The debate over what to do with the prisoners at the Guantanamo Bay detention center got more complicated this week when the FBI foiled an alleged terror plot in New York.

The arrest of four men Wednesday on charges that they plotted to bomb two New York City synagogues and shoot down military airplanes served as a reminder that there are terror cells operating in the United States.

It also provided evidence to some lawmakers that closing the detention center at Guantanamo Bay or releasing any of its 240 detainees into the U.S. federal prison system would further endanger national security.

"The initial reports that we're getting [are] that these alleged terrorists were converted to radical Islam in jails. I think we need to look at that as a problem as well if jails are going to become a breeding ground for people to convert to any kind radical Islam," Rep. Eliot Engel, (D-N.Y.) told FOX News.

Rep. Peter King (R-N.Y.) told FOX News that Islamic radicalization is already a problem in prisons.

"And now if you're bringing hundreds of prisoners from Guantanamo who are very ideologically trained, that will add to it," he said. "And I can tell you, going back several years, whether it's California or New York, there's a real concern about these house conversions because they often become the most radical."

But Frank Cilluffo, director of the Homeland Security Policy Institute at George Washington University, told FOXNews.com that any risk of bringing detainees into the federal prison system can be contained.

"If they had access to the general population, there's huge concern for radicalization," he said. "If they're isolated, I think that concern exists, but I think it can also be handled and addressed."

The arrests of the four New York men this week follow a long line of headline-making homegrown terror plots since Sept. 11, 2001, none of which came close to reality.

But some fear the FBI's winning streak could come to a tragic end if Guantanamo Bay detainees are brought to the U.S.

President Obama has ordered the detention center closed by January, but his timetable may be in jeopardy because the Senate voted overwhelmingly this week to block funding for bringing detainees to the U.S., whether freed or imprisoned.

FBI Director Robert Mueller told Congress on Wednesday that bringing the detainees to the U.S. could pose a number of risks, even if they were kept in maximum-security prisons.

Testifying before the House Judiciary Committee, Mueller was asked what concerns the FBI has about the release of Guantanamo detainees.

"The concerns we have about individuals who may support terrorism being in the United States run from concerns about providing financing, radicalizing others, he said, as well as "the potential for individuals undertaking attacks in the United States.

"All of those are relevant concerns," Mueller said.

Attorney General Eric Holder, asked about Mueller's comments, said the government won't do anything with detainees "that's going to put the American people at risk."

"The concerns that have been expressed by the director, concerns expressed by other people, will all be taken into account," Holder said, adding that the administration is still working on the closure plan and he still believes it can meet the president's deadline.

FOXNews.com's Stephen Clark and Joseph Abrams contributed to this report.