BENGHAZI, Libya (AP) -- There's word of arrests in Libya, in the killing of the U.S. ambassador and three other Americans.

A Libyan official says an unspecified number of militants suspected of taking part in the attack on the U.S. consulate in Benghazi have been arrested. And he says others are being closely monitored by police to see whether they are linked to a group.

The official, a deputy interior minister, also says the attacks were an organized two-part operation by heavily-armed militants. He says the operation included a precisely-timed raid on what was supposed to be a secret safe house, just as Libyan and U.S. security forces were arriving to rescue consulate staff.

According to the Libyan official, the attacks may have been timed to mark the 9/11 anniversary. He says the militants used civilians protesting an anti-Islam film as a cover for their actions.

He also says infiltrators within the security forces may have tipped off militants to the location of the safe house.

The official says Ambassador Chris Stevens and another American, Sean Smith, were killed in the consulate during the initial violence, as plainclothes Libyan security evacuated the staff to the safe house about a mile away.

The second assault took place several hours later and targeted the safe house -- a villa inside the grounds of the city's equestrian club. That attack killed two more Americans. Secretary of State Hillary Rodham Clinton says they were Tyrone S. Woods and Glen A. Doherty, former Navy SEALs who provided security at the consulate.