Washington (CNN) -- Federal authorities investigating the deadly attack on the U.S. diplomatic compound in Benghazi have filed charges against Ahmed Abu 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 just over a month away, the Justice Department has come under criticism for the lack of public progress in the case.
Republican lawmakers have demanded more aggressive steps from the new FBI Director James Comey.
But people close to the case said 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. The agencies are prohibited from discussing matters that are under seal.
It couldn't be learned when the United States plans to announce the charges or whether there are any plans to try to detain Khattalah.
None of the sources would discuss the precise counts against Khattalah in the sealed complaint.
In a recent interview with CNN's Arwa Damon, he acknowledged being at the Benghazi mission after the attack but denied any involvement.
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 investigation is led by agents from the FBI in New York and has included some members of the Washington field office, who have traveled to Libya to interview hundreds of witnesses, those briefed on the investigation said.
Khattalah told CNN that he had not been questioned by either Libyan authorities or the FBI.
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.
The Benghazi attack involved scores of militants using rocket-propelled grenades and other weapons.
Stevens took refuge behind a fortified door with heavy metal bars to keep the attackers from breaking in. But they set fire to the mission -- a villa -- with diesel fuel. Stevens was overcome by smoke.