WASHINGTON (AP) — New Pentagon details show that the first US military unit arrived in Libya more than 15 hours after the attack on the consulate in Benghazi was over, and four Americans, including the ambassador, were dead.

A Defense Department timeline obtained by The Associated Press underscores how far the military response lagged behind the Sept. 11, 2001 attack, due largely to the long distances the commando teams had to travel to get to Libya.

Defense Secretary Leon Panetta and his top military adviser were notified of the attack about 50 minutes after it began and were about to head into a previously scheduled meeting with President Barack Obama. The meeting quickly turned into a discussion of potential responses to the unfolding situation in Benghazi, where militants had surrounded the consulate and set it on fire. The first wave of the attack at the consulate lasted less than two hours.

Ambassador Chris Stevens was killed in the attack. Intelligence, State Department and military officials have released details on the response in an effort to answer Republican criticism that the administration was holding back what and when it knew about the assault.

Panetta and other defense officials have repeatedly said that they did not have armed aircraft or military teams near Benghazi that could have gotten there quickly.

But there have been persistent questions about whether the Pentagon should have moved more rapidly to get troops into Libya or had units closer to the area as the anniversary of the 9/11 attacks on America approached. In particular, there was at least a 19-hour gap between the time when Panetta first ordered military units to prepare to deploy — between midnight and 2 a.m. local time in Tripoli — and the time a Marine anti-terrorism team landed in Tripoli, which as just before 9 p.m.

A senior defense official on Friday said forces were at the ready around the globe, but it took time to assess the murky situation, evaluate the threats, put plans in place and get the teams there. With the situation on the ground rapidly evolving, military officials have said there were a number of potential scenarios that had to be evaluated, including concerns that the violence could continue for some time or there could be a hostage situation to which commandos might have to respond.

In a letter to Republican Sen. John McCain on Friday, Panetta specifically addressed the claim that the military could have dispatched armed unmanned aerial vehicles, AC-130 gunships or fighter jets to thwart the attack. Such aircraft were not in the region and not an effective option, he said.

Panetta said that based on a continuous evaluation of threats, military forces were spread around Europe and the Middle East to deal with a variety of missions. In the months before the attack, he noted, “several hundred reports were received indicating possible threats to US facilities around the world” and noted that there was no advance notice of imminent threats to US personnel or facilities in Benghazi.

His explanation, however, did not satisfy McCain. In a statement Friday, McCain said Panetta’s letter, “only confirms what we already knew — that there were no forces at a sufficient alert posture in Europe, Africa or the Middle East to provide timely assistance to our fellow citizens in need in Libya. The letter fails to address the most important question — why not?”

The attack began at about 9:40 p.m. local time in Benghazi. Less than 20 minutes later, the US military began moving an unarmed drone to a position over Benghazi, so it could provide real time intelligence to the CIA team on the ground. The CIA team went to aid the Americans at the consulate. The drone arrived shortly after 11 p.m. By 11:30 p.m., a CIA team was able to get all the Americans out of the compound.

As that was happening, Panetta and Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, left the Oval Office and went into a series of meetings in the Pentagon with senior leaders to discuss how to respond to the Benghazi attack and assess the potential for other outbreaks of violence in the region.

Between midnight and 2 a.m., Panetta began to issue verbal orders, telling two Marine anti-terrorism teams based in Rota, Spain, to prepare to deploy to Libya, and he ordered a team of special operations forces in Central Europe and another team of special operations forces in the US to prepare to deploy to a staging base in Europe.

As the military units begin moving, just before dawn, the Americans in Benghazi, who were now at the CIA base less than a mile away from the consulate, again came under attack around 5:15 a.m. when five mortars were fired at the building. Two missed, but three hit, killing two CIA security officers who were on the roof.

The Americans fired back and soon afterward fled the CIA base for the airport. By 10 a.m., they had flown out, heading to Tripoli. Shortly after 7 p.m., the Americans, including the bodies of the four dead, were flown out of Tripoli on a military aircraft.

Not until just before 8 p.m., however, did the first US military unit arrive in the region, as the special operations team landed at Cigonella Naval Air Station in Sicily. An hour later, the Marine team landed in Tripoli. The defense official noted that even if the military had been able to get units there a bit faster, there was no way they could have gotten there in time to make any difference in the deaths of the four Americans.

“The US Armed Forces did everything they were in position to do to respond to the attack in Benghazi,” Panetta said in the letter, obtained by The Associated Press. “The department’s senior leaders and I spared no effort to save the lives of our American colleagues, as we worked to bolster security in response to a series of other threats in the region occurring at the same time.”

Copyright 2012 The Associated Press.