FBI spokeswoman declined to say what was found during one-day visit to Libya as issue continues to permeate election

A team of FBI agents has travelled to the Libyan city of Benghazi to investigate the terror attack on the US consulate last month that killed the US ambassador, according to officials.

Agents arrived in Benghazi on Wednesday and departed Thursday after weeks of waiting for access to the crime scene to investigate the attack that killed a state department computer expert and two security agents as well as ambassador Chris Stevens. FBI spokeswoman Kathy Wright said the agents went to "all the relevant locations" in Benghazi, but would not say what, if anything, they found.

US officials said the hunt for the attackers had narrowed to one or two people in a local extremist group who may have had something to do with the attack.

The attack, by what is now believed to be al-Qaida-linked militants, has become fraught with election-year politics as Republicans accuse administration officials of being misleading in the early aftermath on what they knew about the perpetrators and for lax security at the diplomatic mission in a lawless part of post-revolution Libya. Immediately after the attack, officials said the consulate was stormed by protesters outraged over an anti-Muslim film produced by a California man.

US intelligence and special operations forces have focused on at most "one or two individuals" in the Libya-based extremist group Ansar al-Shariah who may have had something to do with the attack, according to a US counterterrorism official. But that official and two others said there was no definitive evidence linking even those individuals to the attack. The officials spoke on condition of anonymity because they were not authorized to comment on the investigation publicly.

Members of Ansar al-Shariah were recorded making boastful calls to other militants after the attack, including to members of al-Qaida in the Islamic Maghreb, which is suspected of having a role in the attack, one of the officials said. But that's common in the aftermath of any such incident, when different militant groups try to claim credit to build their own stature in the region, the official said.

So far, US intelligence has found no evidence showing communication between militants prior to the attack, which took place on the 11th anniversary of the September 11 terror attacks on the US.

Several Republican lawmakers have said Stevens and his staff made repeated requests for security improvements at the Benghazi consulate that the state department denied.

The state department has assigned an independent panel to look into the security procedures before, during and after the attack. That five-member accountability review board met for the first time Thursday and compiled documents to go through, state department spokeswoman Victoria Nuland said. The board must submit its findings and any recommendations it may have to secretary of state Hillary Clinton within 60 days, unless it is determined that more time is required.

The Pentagon is conducting its own internal review to see whether the military played any role in assessing the security in Libya, spokesman George Little said.

FBI agents had been staying away from Benghazi until the city was more secure, law enforcement officials said. But agents were in other parts of the country investigating the attack since 18 September.

Little said it was "a matter of days" between the request for the FBI to access the Benghazi crime scene and the team's arrival Wednesday when the US military airlifted them to the city.

Attorney general Eric Holder said people should not assume that "all that we could do or have been doing" in the investigation is restricted solely to Benghazi.

"I'm satisfied with the progress," Holder said Thursday. He said there were a variety of other places inside and outside Libya where "all these things could be done and have been done and that the matter has been under active investigation".