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WASHINGTON — Federal criminal charges have been filed against one or more suspects in the lethal attack on the U.S. diplomatic mission in Benghazi, Libya, last year, two law enforcement sources said Tuesday.

One of the sources said charges were filed under seal against Ahmed abu Khattala, a top Libyan militia commander who has admitted being at the scene of the nightlong attack by armed extremists on Sept. 11.

Ambassador J. Christopher Stevens, an aide and two CIA contractors died in assaults on two U.S. compounds.

The sources declined to discuss details because the case is under seal.

Justice Department spokesman Andrew Ames said in a statement that "the department's investigation is ongoing."

"It has been, and remains, a top priority. We have no further comment at this time."

The Obama administration has been under increasing political pressure, primarily from Republicans, to hold someone accountable for the attack. Critics charge that the Justice Department and FBI have not made a concerted effort to make arrests.

Much of the diplomatic compound was looted and trampled, making the crime scene difficult to analyze, and FBI Director Robert S. Mueller III told the House Judiciary Committee in June that U.S. agents could not immediately get to the area because of safety issues.

"We've had some success that I can't get into today," Mueller said at the time.

In May, Atty. Gen. Eric H. Holder Jr. told the same House panel that results of the Benghazi investigation would be made public soon.

"We will be prepared shortly, I think, to reveal all that we have done," he said.

Holder added, "I'm satisfied with the progress that we have made in the investigation. Regardless of what happened previously, we have made very, very, very substantial progress."

U.S. officials have said that Abu Khattala, an Islamic cleric and reputed leader of the extremist Ansar al Sharia militia, was a leading suspect in the attack. He told reporters in October that he was present during the attack but denied that he was directing the militants.

Ali Ani Harzi was arrested in October in Turkey in connection with the attack at the request of the CIA and sent back to his home country, Tunisia. FBI agents later interviewed Harzi for three hours in front of a local judge, but he was released from custody Jan. 8 for lack of evidence. Then-Secretary of State Hillary Rodham Clinton told Congress that the Tunisian prime minister had assured her that Harzi remained "under the monitoring of the court."

A third suspect, Islamist militant Muhamed Jamal abu Ahmed, was arrested in Egypt in December with help from U.S. intelligence officials. They believe some of Ahmed's associates were present during the Benghazi attack, although his own role remains unclear.

Rep. Darrell Issa (R-Vista), who as chairman of the House Oversight and Government Reform Committee has been pursuing answers in the Benghazi case, said Tuesday that anyone involved should be "questioned and placed in custody of U.S. officials without delay."

He noted that Osama bin Laden, the late leader of Al Qaeda, was criminally indicted in U.S. courts long before the Sept. 11, 2001, attacks and added that "delays in apprehending the suspected Benghazi killers will only put American lives at further and needless risk."

U.S. intelligence officials have said that even though some Al Qaeda extremists were present at the Benghazi attack, it was not a premeditated Al Qaeda operation. According to the State Department, Stevens died of smoke inhalation in a so-called safe room after armed militants stormed the lightly guarded diplomatic compound and set several buildings on fire.

Sean Smith, a State Department communications specialist, was killed in another room. Glen Doherty and Tyrone Woods, two former Navy SEALs working as security contractors for the CIA, were killed the next morning when a mortar round hit their position at a CIA base about a mile from the compound.

richard.serrano@latimes.com

ken.dilanian@latimes.com