Story highlights FIRST ON CNN: Charges filed against Libyan militia leader Ahmed Abu Khattala, others

Attack last September killed Ambassador Christopher Stevens and three other Americans

Authorities would not detail counts as charges remain under seal

The Benghazi attack has become a political flashpoint in Washington

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 other Americans 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.

Photos: Photos: Attack on U.S. mission in Benghazi Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Attackers set the U.S. mission in Benghazi, Libya, on fire on September 11, 2012. The U.S. ambassador to Libya, Christopher Stevens, and three other U.S. nationals were killed during the attack. The Obama administration initially thought the attack was carried out by an angry mob responding to a video, made in the United States, that mocked Islam and the Prophet Mohammed. But the storming of the mission was later determined to have been a terrorist attack. Hide Caption 1 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Obama and Clinton stand at Andrews Air Force Base as the bodies of the four Americans killed are returned on September 14. Hide Caption 2 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A desk sits inside the burnt U.S. mission on September 13, two days after the attack. Hide Caption 3 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Damage is seen inside the U.S. mission on September 13. Hide Caption 4 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A lounge chair and umbrella float in the swimming pool of the U.S. mission on September 13. Hide Caption 5 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Demonstrators gather in Libya on September 12 to condemn the killers and voice support for the victims. Hide Caption 6 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – U.S. President Barack Obama, with Secretary of State Hillary Clinton on September 12, makes a statement at the White House about Stevens' death. Hide Caption 7 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A burnt vehicle is seen at the U.S. mission in Benghazi on September 12. Hide Caption 8 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – People inspect the damage on September 12. Hide Caption 9 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A small American flag is seen in the rubble on September 12. Hide Caption 10 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A man stands in part of a burned-out building of the U.S. mission on September 12. Hide Caption 11 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Smoke and fire damage is evident inside a building on September 12. Hide Caption 12 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Half-burnt debris and ash cover the floor of one of the U.S. mission buildings on September 12. Hide Caption 13 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – The U.S. mission is seen in flames on September 11, the day of the attack. Hide Caption 14 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A protester reacts as the U.S. mission burns on September 11. Hide Caption 15 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A vehicle and the surrounding area are engulfed in flames on September 11. Hide Caption 16 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Flames erupt outside of a building on September 11. Hide Caption 17 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A vehicle burns during the attack on the U.S. mission on September 11. Hide Caption 18 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Onlookers record the damage from the attack on September 11. Hide Caption 19 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Onlookers walk past a burning truck and building on September 11. Hide Caption 20 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A vehicle sits smoldering in flames on September 11. Hide Caption 21 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – People duck flames outside a building on September 11. Hide Caption 22 of 22

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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 them that the Justice Department would soon make public what actions it would take.

Republicans 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.