WASHINGTON - A team of FBI agents arrived in Benghazi, Libya, to investigate the assault against the U.S. Consulate and left after about 12 hours on the ground as the hunt for those possibly connected to the attack that killed Ambassador J. Christopher Stevens and three other Americans narrowed to one or two people in an extremist group, U.S. officials said Thursday.

Agents arrived in Benghazi before dawn on Thursday and departed after sunset, after weeks of waiting for access to the crime scene to investigate the Sept. 11 attack.

The agents and several dozen U.S. special operations forces were there for about 12 hours, said a senior Defense Department official who spoke anonymously because he was not authorized to speak publicly about the ongoing investigation. The FBI agents went to "all the relevant locations" in the city, FBI spokeswoman Kathy Wright said. The FBI would not say what, if anything, they found.

Killed in the attack were Stevens, a State Department computer expert, and two security agents who were former Navy SEALS. Al-Qaeda-linked militants are believed responsible.

In the United States, the attack has become caught up in election-year politics. Republicans accuse Obama administration officials of being misleading in the early aftermath about what they knew about the attackers and for lax security at the diplomatic mission.

Immediately after the attack, officials said the consulate was stormed by protesters outraged over an anti-Muslim film produced by a California man.

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