TRIPOLI, Libya (AP) — The FBI director met with top Libyan officials on Thursday to discuss the probe into last year's killing of the U.S. ambassador in Benghazi where authorities are planning a curfew following an upsurge in violence, Libyan officials said.

Ambassador Chris Stevens and three other Americans were killed on Sept. 11, 2012 in an attack that Washington officials suspect was carried out by militants linked to the al-Qaida terrorist group. There has been little news of progress in the investigation, and U.S. officials have complained about poor cooperation with governments in the region.

The officials, who spoke on condition of anonymity because of the sensitive nature of the visit, said FBI Director Robert Mueller discussed the case in Tripoli with senior officials, including the prime minister, justice minister and intelligence chief.

It is unclear when authorities plan to impose a curfew following a string of deadly attacks, assassinations of top security officials and other unrest in recent months. Libyan Prime Minister Ali Zidan said on Wednesday that 18,000 new police recruits would be dispatched to the city to enforce the curfew.

Interior Minister Ashour Shaweil told reporters that when it starts, it will be enforced for five hours every night beginning at midnight. Several check points will be installed around the city, he said.

Authorities in Libya have been struggling to form unified army and police force since 2011 when former Libyan dictator Moammar Gadhafi was ousted and killed in a mass uprising that turned to armed conflict when citizens raised arms against Gadhafi's forces.

In recent months, there has been a series of assassinations of top security officials and bombings of security headquarters in the northeastern city of Benghazi. Some blame Islamist extremists, but residents suspect more than one group is involved and that some of the violence is being carried out by those who have personal vendettas against officials who once served in Gadhafi's police force.