Four House Republicans formally requested an investigation into the Council on American Islamic Relations this week after charging that the nation's largest Muslim advocacy group tried to "infiltrate" Capitol Hill with interns. 

The lawmakers based most of their claims on an internal memo and the book "Muslim Mafia," which describes CAIR's purported strategy to influence members of Congress on the Judiciary, Intelligence and Homeland Security Committees. They raised concerns about CAIR in large part because the Justice Department named the group as an unindicted co-conspirator in a terrorism case. 

In a letter to Sergeant-At-Arms Wilson Livingood, the lawmakers urged him to review the group's alleged ties to Hamas and "determine if CAIR is a security threat." They also asked that he determine whether the group was successful in placing interns on key House panels. 

"If you find that CAIR is a security threat, we ask that you take the appropriate security measures," they wrote. 

The Intelligence Committee does not accept interns, and the Homeland Security and Judiciary Committees select candidates on a case-by-case basis with no preferential treatment given to any group, congressional aides told Foxnews.com. 

The Homeland Security Committee, which was made permanent only four years ago, has not had any interns sponsored by CAIR in at least three years, an aide to the panel said, adding that he could not confirm whether a CAIR intern had ever assisted the panel. It's also not clear whether any candidates sponsored by CAIR applied during that period, the aide said. 

The Judiciary Committee has not had any CAIR interns since May, an aide said. 

Reps. Sue Myrick of North Carolina, Trent Franks of Arizona, Paul Broun of Georgia and John Shadegg of Arizona raised serious concerns about the group and wrote to Attorney General Eric Holder urging him to provide Congress with a summary of why the group was named as a co-conspirator. 

But other lawmakers have rushed to the organization's defense. A CAIR spokesman accused the Republican representatives of being "Muslim bashers" by trying to exclude Muslims from the political process.