U.S. counterterrorism officials have determined that several extremist groups, including Ansar al-Sharia, took part in last year’s attack in Benghazi, Libya, that killed a U.S. ambassador and three other officials. They think the terrorist organizations selected the U.S. diplomatic outpost there as a potential target ahead of time.

The officials have identified numerous people involved — some new to U.S. intelligence and others who are well-known — and have issued several sealed indictments in recent months.

But on the first anniversary of the attack, there is not “anyone in custody who can tell us” specifics, including when, where and by whom the plot was hatched, and whether the Sept. 11 date was selected in advance or was a last-minute choice of opportunity, a counterterrorism official said.

“That is a huge gap,” said the official, who spoke on the condition of anonymity to discuss the investigation. “What we lack is a source of information that puts us where we need to be.”

Although the Benghazi attack has been overshadowed by political and foreign policy crises since hearings on the matter were held earlier this year, it remains a live issue for federal investigators and for House Republicans determined to prove that the Obama administration is covering up failures that occurred before, during and after the assault.

Libyan military guards check one of the U.S. consulate's burnt-out buildings after the death of American ambassador J. Christopher Stevens and his colleagues in the Sept. 11, 2012, attack in Benghazi, Libya. (Mohammad Hannon/Associated Press)

Throughout the summer, House Oversight Committee Chairman Darrell Issa (R-Calif.) has engaged in increasingly acrimonious exchanges with the State Department and the independent Accountability Review Board that it appointed to investigate. He issued two subpoenas for documents last month and, with the support of Speaker John A. Boehner (R-Ohio), plans to hold additional hearings this fall.

“What we’ve learned in a year is that stalling, delay and information that goes unchallenged is part of how you make it an old story to make it go away,” Issa said in an interview last week, adding that the administration “didn’t heed the warnings in advance, didn’t respond . . . or even try to” once the attack on the diplomatic facility and a nearby CIA annex began, and “made false statements” in its aftermath.

On Tuesday, Issa sent a letter to Secretary of State John F. Kerry complaining about the State Department’s unwillingness to facilitate testimony by two Diplomatic Security agents who were at the CIA facility in Benghazi during the attack, one of whom was severely wounded and remained hospitalized this summer.

Kerry is clearly irritated by GOP pummeling of what he considers an issue that already has been examined exhaustively. “I do not want to spend the next year coming up here talking about Benghazi,” he told a House committee in April. Last week, he snapped at a Republican lawmaker who interjected Benghazi accusations into a House briefing on Syria.

Democratic lawmakers have expressed equal exasperation. Rep. Elijah E. Cummings (Md.), the Oversight Committee’s ranking minority member, said Issa should concentrate on ensuring that recommendations to avoid such attacks are fully carried out “rather than trying to pick a needless partisan fight.”

Instability and violence

U.S. personnel in Libya no longer travel to Benghazi, where a volatile stew of extremist groups and militias holds sway and bombings and shootings are the norm. Most recently, the Libyan military’s chief prosecutor and his brother were killed Aug. 29 by a car bomb that was detonated remotely.

The rest of Libya is similarly violent and unstable. In the capital, Tripoli, diplomats, FBI investigators and other officials live and work in the former residence of the U.S. ambassador. The 11-acre compound near the international airport is surrounded by tall blast walls and heavily guarded by local security forces. U.S. Marines man a large, camouflage-covered gun in a tower.

U.S. Ambassador Deborah Jones works in a small office that was once a staff bedroom. Overall staffing has been significantly reduced since the Benghazi attack, and Libyans seeking U.S. visas must now apply remotely through the U.S. Consulate in Casablanca, Morocco.

For the FBI team in Tripoli, life and work are frustrating. The instability, the violence and the lack of government control over the country make the investigation “far different than anything” U.S. law enforcement has ever attempted, including investigations of the 1998 U.S. Embassy bombings in Kenya and Tanzania, the counterterrorism official said.

Although U.S. officials praise the Libyan government’s willingness to cooperate, they acknowledge that the government is not in charge of much.

The FBI Web site has posted dozens of photographs of people, most taken from videos on the day of the attack, whom the bureau would like to speak with in Libya. With little ability to work on the ground, the FBI has made what it considers innovative and unprecedented use of Facebook and other social-media outlets to trace and talk to potential sources of information.

Using overhead drones and other types of surveillance, FBI agents in Tripoli, Washington and elsewhere try to keep close watch on suspects, hoping that they will travel to another country — preferably one with a U.S. extradition treaty — where they can be arrested.

Some of them are easy to find in Libya. Ahmed Abu Khattala, a prominent militia leader in Benghazi and the subject of a sealed U.S. indictment in the killing of U.S. Ambassador J. Christopher Stevens and the three others, has been frequently interviewed by U.S. media, most recently by CNN in late July.

But such contact is “not so easy as some might think” for the FBI, the official said. U.S. law enforcement has no authority to make arrests in a foreign country.

Ill-equipped for response

Despite significant resources based in southern Europe, the U.S. military was ill-positioned — and unwilling, some critics say — to swiftly respond to the Benghazi assault as it was happening. Although the military is still limited by what the Pentagon calls “the tyranny of distance,” with personnel and equipment relatively far away from trouble spots in Africa and the Middle East, it has made some significant changes.

The Defense Department has upgraded its crisis management coordination with the State Department to conduct a daily review of intelligence on emerging threats around the world and decide whether and where increased security is necessary.

“In the past . . . we were by design reactive to respond to crises out there,” said a senior military official who was authorized to discuss the issue on the condition of anonymity. Now, the official said, “we are in position to proactively get out in front of it.”

“Tangibly, what the layman would see is a significant increase in Marine security guards at high-threat, high-risk” diplomatic posts, the military official said. A new 500-Marine crisis response team stationed at Morón Air Base in Spain and increased resources around the Mediterranean have been given timelines for rapid response. The State Department has hired 113 new Diplomatic Security personnel, including 75 agents.

In the past four months, as crises have erupted and terrorist threats led the department to temporarily close embassies and consulates in 20 countries, the official said, military forces have been moved around to respond quickly “70 to 80 times.”

Military teams have been repositioned directly to crisis countries “a couple dozen times to help that embassy team or country team given in front of a problem set,” the official said.

Issa said he considers what the military has done the “good news” of the Benghazi fallout.

Most of his ire remains focused on the White House and the State Department, which he said continues to stonewall requests for documents and testimony. He has accused the State Department and the CIA of dissuading officials from talking to the committee and of giving polygraph tests to officials to make sure no one is talking — allegations that both categorically deny.

Issa has called the Accountability Review Board report, which was released in an unclassified version in December, a “whitewash.” But he has not scheduled the public hearing demanded by Thomas Pickering, the board’s chairman, to answer his charges.

This summer, Issa subpoenaed all of the board’s internal work, including transcripts of closed-door interviews, notes, internal communications of members and report drafts.

In a six-page, single-spaced response asking Issa to reconsider the subpoena, Thomas B. Gibbons, the acting assistant secretary of state for legislative affairs, was barely able to contain his irritation.

“The Department has taken part in more than 50 briefings and in nine congressional hearings,” made 10 officials available for full-day interviews and supplied 25,000 documents, Gibbons said in the Aug. 23 letter, a copy of which The Washington Post obtained.

“Particularly in light of recent security threats that necessitated the temporary closing of embassies and consulates in the Middle East,” he wrote in closing, “the Department cannot afford to have its resources and attention diverted from this critical work.”

Kevin Sullivan in Tripoli contributed to this report.