An unspecified number of militants suspected of taking part in an attack on the American consulate in Benghazi, Libya, have been arrested and others are being closely monitored by police to see whether they are linked to a group, a senior Libyan security official has said.

Wanis el-Sharef, eastern Libya's deputy interior minister, announced the arrests Thursday but refused to elaborate.

The exact number of arrests made remains unclear. El-Sharef told Reuters Thursday that four suspects were in custody, while Libyan Prime Minister Mustafa Abushagur told CNN that one arrest had been made earlier that day in Benghazi, and that three or four other suspects were still being pursued. He said that the person arrested and the people being sought were all Libyans.

The attack that killed four Americans, including the U.S. ambassador, was an organized two-part operation by heavily armed militants that included a precisely timed raid on a supposedly secret safe house just as Libyan and U.S. security forces were arriving to rescue evacuated consulate staff, El-Sharef said.

The attacks Tuesday night were suspected to have been timed to mark the Sept. 11 anniversary and that the militants used civilians protesting an anti-Islam film as cover for their action. Infiltrators within the security forces may have tipped off militants to the safe house location, he said.

Ambassador Chris Stevens and another American were killed in the consulate during the initial violence, as plainclothes Libyan security were evacuating the consulate's staff to the safe house about a mile away, el-Sharef said. The second assault took place several hours later and targeted the safe house — a villa inside the grounds of the city's equestrian club — killing two Americans and wounding a number of Libyans and Americans.

Escalating series of events

El-Sharef, who was running the Interior Ministry's operations room commanding security forces in the city, gave The Associated Press an account of the night's chaotic events.

The crowd built at the consulate — a one-story villa surrounded by a large garden in an upscale Benghazi neighbourhood — in several stages, he said. First, a small group of gunmen arrived, then a crowd of civilians angry over the film. Later, heavily armed men with armoured vehicles, some with rocket-propelled grenades, joined, swelling the numbers to more than 200.

The gunmen fired into the air outside the consulate. Libyan security guarding the site pulled out because they were too few. "We thought there was no way for the protesters to storm the compound, which had fortified walls," he said. El-Sharef said Libyan security advised the Americans to evacuate at that point, but he says the advice was ignored. There was shooting in the air from inside the consulate compound, he said.

At this point, he said, the crowd stormed the compound. The consulate was looted and burned, while plainclothes security men were sent to evacuate the personnel.

Stevens, he said, is likely to have died of asphyxiation following a grenade explosion that started a fire, el-Sharef said. Ziad Abu Zeid, a Libyan doctor to whom Stevens' body was taken, told The Associated Press on Wednesday that the 52-year-old Stevens died of asphyxiation.

U.S. officials have said attackers broke into the main consulate building at around 10:15 p.m. and set the compound on fire. Amid the evacuation, Stevens became separated from others, and staffers and security that tried to find him were forced to flee by flames, smoke and gunfire. After an hour, according to U.S. officials, U.S. and Libyan officials drove the attackers from the consulate.

2nd attack hours later

The next attack came hours later. Around 30 American staffers along with Libyans had been evacuated to the safe house while a plane arrived from Tripoli with a joint U.S.-Libyan security group that was to fly them back to the capital, el-Sharef said.

El-Sharef said the original plan was for a separate Libyan security unit to escort the evacuees to the airport. Instead, the joint unit went from the airport to the safe house, possibly because they were under the impression they were dealing with a hostage situation, he said. The militant attack coincided with the joint team's arrival at the safe house, he said.

That the attackers knew the safe house's location suggests a "spy" inside the security forces tipped off the militants, he said.

U.S. officials have not confirmed the account. They have spoken of an attack on the consulate's annex that killed two Americans, but said their report on the incident was still preliminary.

Victims identified

Air Force veteran Sean Smith, who worked as an information management specialist for the State Department, was identified Tuesday as one of the consulate employees killed in the attack.

The State Department on Thursday also named the other two Americans who were killed as Tyrone S. Woods and Glen A. Doherty, former Navy SEALs who provided security at the consulate.

U.S. President Barack Obama, speaking a campaign event in Golden, Colo., vowed that the perpetrators would be punished.

"I want people around the world to hear me," he said. "To all those who would do us harm: No act of terror will go unpunished. I will not dim the light of the values that we proudly present to the rest of the world. No act of violence shakes the resolve of the United States of America."

Prime Minister Stephen Harper also denounced the attacks, calling them "horrific" and "disgusting."

In an interview that aired Thursday on Byline With Brian Lilley on Sun News Network, Harper expressed his condolences to the friends and families of the victims.

"What happened is extremely disturbing. It's horrific, disgusting," Harper said.

"Diplomats do not sign up for military service. We assume that our diplomats can conduct the business of their respective countries free from fear of persecution or violence."

With files from The Canadian Press