Federal authorities have filed the first criminal charges in the investigation of the deadly terror attack on the U.S. diplomatic compound in Benghazi, Libya, people briefed on the matter said.

Several suspects charged in the armed assault last September 11 include prominent Libyan militia figure Ahmed Abu Khattala, the sources said. Other identities were not disclosed.

The counts initially sought months ago in New York are still under seal, according to the sources.

The investigation of the attack that killed Ambassador Christopher Stevens and three others continues, these people said, as investigators try to build their case against Khattala and others who authorities believe were involved.

Justice Department spokesman Brian Fallon declined to comment as did FBI spokesman J. Peter Donald in New York.

But word of the development comes as the Justice Department has come in for sharp criticism from congressional lawmakers about the pace and progress of the investigation.

Attorney General Eric Holder had earlier this year promised congressional lawmakers the Justice Department would soon make public what actions it would take.

Republican lawmakers have demanded more aggressive steps from the incoming FBI Director James Comey.

But people close to the case said the investigation has moved significantly in recent months.

In that time, the FBI has released photos of what it called persons of interest who were present during the attacks, seeking tips from the Libyan public.

It couldn't be learned when the United States plans to announce the charges or whether there is any intention to try to detain Khattala or the others.

None of those briefed on the matter would discuss the precise counts.

In a recent interview with CNN's Arwa Damon, Khattala 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.

Khattala 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.

U.S. Rep. Adam Schiff, a Democratic member of the House Intelligence Committee, said in an interview on "The Lead with Jake Tapper" that he would not describe the investigation as proceeding rapidly.

"It's been frustrating for many of us that it hasn't moved faster, but it is a very difficult working environment for our agents," said Schiff. "Gathering evidence in Libya, you might imagine, is very difficult, finding, interviewing witnesses, extremely difficult."

But Schiff said there has been progress.

"We have identified many of the parties involved. We're still trying to identify what the command-and-control structure would be. A lot of missing pieces still, but we are finally making progress," said Schiff.