Washington (CNN) -- Federal agents and prosecutors investigating the attack on the U.S. diplomatic compound in Benghazi have filed charges against Ahmed Khattalah, leader of a Libyan militia that officials believe was involved in the assault, people briefed on the investigation said. The charges under seal are the first criminal counts to emerge from the probe.
The investigation of the attack last September 11 that killed Ambassador Christopher Stevens and three others continues, these people said, as investigators try to build their case against Khattalah and others who authorities believe were involved.
Attorney General Eric Holder had earlier this year promised congressional lawmakers the Justice Department would soon make public what actions it would take.
With the anniversary of the attack looming, the Justice Department has come under criticism for the lack of public progress in the case.
People close to the case say the investigation has moved significantly in recent months. The initial charges are filed in a complaint in New York, according to the people briefed on the matter.
The Justice Department and the FBI declined to comment on the charges. The agencies prohibit commenting on matters that are under seal. It couldn't be learned when the U.S. plans to announce the charges and whether there are any plans to try to detain Khattalah.
None of the sources would discuss the precise charges against Khattalah in the sealed complaint.
Benghazi has become a political flashpoint in a long-running battle between the Obama administration and Republicans, who accuse it of not bolstering security before the attack, of botching the response to it and of misleading the public for political gain less than two months before the November election.
The probe is led by agents from the FBI in New York, and has included some members from the agency's Washington field office who have traveled to Libya to interview hundreds of witnesses, those briefed on the investigation said.
The Benghazi attack involved scores of militants using rocket-propelled grenades and other weapons. They quickly overwhelmed a lightly guarded compound where Stevens had turned in for the night. The attack resumed later that night after American survivors evacuated to the nearby CIA compound,
Khattalah has kept a public presence, including granting interviews to CNN.
U.S. law enforcement officials said it's not unusual for the FBI not to seek to interview defendants in a case while they collect other evidence.