Federal law enforcement authorities have filed murder charges against Ahmed Abu Khattala, a prominent militia leader in Benghazi, Libya, in connection with the attacks on a diplomatic mission there last Sept. 11 that killed the United States ambassador and three other Americans, according to senior law enforcement and United States officials.

The authorities have identified roughly a dozen others who they said they believe participated in the attacks, and have filed charges under seal against some of them, the officials said.

Despite making progress in the investigation, some F.B.I. agents who are leading it from Tripoli, the capital of Libya, have grown frustrated that there have been no arrests, the officials said. Apprehending the suspects will most likely take significant negotiations between the State Department and the Libyan government over who will try to do so and where the suspects will be tried.

It is not clear that either government knows the whereabouts of all the suspects.

“It is a very difficult environment to work in,” one United States official said. “It’s a fragile country.”