A former U.S. Air Force counterintelligence specialist who defected to Iran has been charged with revealing classified information as well as research about her former colleagues to representatives of the Tehran government, prosecutors said Wednesday.

A Justice Department indictment charges Monica Elfriede Witt, who defected in 2013 and is currently at-large, along with four Iranian hackers who, prosecutors say, used the information she provided to target former colleagues in the U.S. intelligence community.

The indictment says the four Iranians were acting on behalf of the government-linked Iranian Revolutionary Guard. All four remain at large.

"It is a sad day for America when one of its citizens betrays our country," said Assistant Attorney General John Demers, the head of the Justice Department's national security division.

Jay Tabb, the FBI's top national security official, said the FBI had warned Witt before her defection that she was a vulnerable target for recruitment by Iranian intelligence but that Witt had ignored those warnings.