WASHINGTON -- Security officers from the C.I.A. played a pivotal role in combating militants who attacked the American diplomatic mission in Benghazi, Libya, on Sept. 11, deploying a rescue party from a secret base in the city, sending reinforcements from Tripoli, and organizing an armed Libyan military convoy to escort the surviving Americans to hastily chartered planes that whisked them out of the country, senior intelligence officials said Thursday.
The account given by the senior officials, who did not want to be identified, provided the most detailed description to date of the C.I.A.'s role in Benghazi, a covert presence that appears to have been much more significant than publicly disclosed.
Within 25 minutes of being alerted to the attack against the diplomatic mission, half a dozen C.I.A. officers raced there from their base about a mile away, enlisting the help of a handful of Libyan militia fighters as they went. Arriving at the mission about 25 minutes after that, the C.I.A. officers joined State Department security agents in a futile search through heavy smoke and enemy fire for Ambassador J. Christopher Stevens before evacuating the mission's personnel to the apparent safety of their base, which American officials have called an annex to the mission. Mr. Stevens was one of four Americans killed in the attack.
A four-hour lull in the fighting beginning shortly after midnight seemed to suggest that the worst was over. An unarmed military drone that the C.I.A. took control of to map possible escape routes relayed reassuring images to Tripoli and Washington. But just before dawn, and soon after a C.I.A.-led team of reinforcements, including two military commandos, arrived from Tripoli, a brief but deadly mortar attack surprised the Americans. Two of the C.I.A. security officers who were defending the base from a rooftop were killed.
"The officers on the ground in Benghazi responded to the situation on the night of 11 and 12 September as quickly and as effectively as possible," one of the senior intelligence officials told reporters.
Thursday's briefing for reporters was intended to refute reports, including one by Fox News last Friday, that the C.I.A.'s chain of command had blocked the officers on the ground from responding to the mission's calls for help.
"There were no orders to anybody to stand down in providing support," the official said, speaking on the condition of anonymity because of continuing investigations by the State Department and the F.B.I.
At a time when the circumstances surrounding the attack on the Benghazi compound have emerged as a major political issue, with Republicans criticizing the Obama administration's handling of the episode, the senior official also sought to rebut reports that C.I.A. requests for support from the Pentagon that night had gone unheeded.
In fact, the official said, the military diverted a Predator drone from a reconnaissance mission in Darnah, 90 miles away, in time to oversee the mission's evacuation. The two commandos, based at the embassy in Tripoli, joined the reinforcements. And a military transport plane flew the wounded Americans and Mr. Stevens's body out of Libya.
Despite the new details, many questions surrounding the attack remain unanswered, including why the State Department did not increase security at the mission amid a stream of diplomatic and intelligence reports that indicated that the security situation in Benghazi and around Libya had deteriorated sharply since the United States reopened its embassy in Tripoli last year.
By underscoring the C.I.A.'s previously unpublicized role in mobilizing the evacuation effort, the officials seemed to be implicitly questioning the State Department's security arrangements in Benghazi, a focus of three Congressional inquiries into the attack on the mission.
The senior officials also shed new light on the C.I.A.'s role in Libya.
Within months of the start of the Libyan revolution in February 2011, the agency began building a meaningful but covert presence in Benghazi, a locus of the rebel efforts to oust the government of Col. Muammar el-Qaddafi.
The C.I.A.'s surveillance targets in Benghazi and eastern Libya included Ansar al-Shariah, a militia that some have blamed for the attack on the mission, as well as suspected members of Al Qaeda's affiliate in North Africa, known as Al Qaeda in the Islamic Maghreb.
American intelligence operatives also helped State Department contractors and Libyan officials in tracking shoulder-fired missiles taken from the former Libyan Army arsenals, American officials said.
The C.I.A.'s security officers played a new role on Sept. 11, carrying out an informal agreement with the mission to come to its aid in an emergency. One of the senior intelligence officials provided an hour-by-hour chronology of the agency's role during the attack.
Around 9:40 p.m. local time, the C.I.A. base received the first of several calls from the mission saying it was under attack. During the 25 minutes between the first call and when the officers rolled out the door, half a dozen security officers were readying their gear and weapons, while the base chief called several Libyan militias, seeking fighters with heavy weaponry to defend the mission. His appeals failed.
Over the next 25 minutes, C.I.A. officers approached the walled diplomatic compound, tried to secure heavy weapons, and made their way onto the compound itself in the face of enemy fire.
At 11:11 p.m., the Predator drone arrived over the mission compound. Within 20 minutes, all United States personnel, except for Mr. Stevens, whom the American security officers could not find in the chaos, left the mission, coming under fire as they did.
The Americans retreated safely to the C.I.A. annex, where over the next 90 minutes they came under sporadic small-arms fire and rocket-propelled-grenade attacks. The State Department and C.I.A. officers returned fire and the assailants melted away.
About this same time, the reinforcements arrived at the Benghazi airport from Tripoli. Learning that the attacks at the annex had stopped, the team turned its attention to finding Mr. Stevens. But learning that he was at a Benghazi hospital, almost certainly dead, and that the security situation at the hospital was uncertain, the reinforcements headed to the annex.
They arrived shortly after 5 a.m., just before mortar rounds began to hit the annex. That attack, 11 minutes long, killed two men, whom the senior intelligence officials identified for the first time Thursday as C.I.A. security officers, Tyrone S. Woods and Glen A. Doherty, former members of the Navy SEALs. Until now the men had been publicly identified as State Department contract security officers.
Less than an hour later, a convoy of 50 heavily armed trucks from Libyan military intelligence arrived to help evacuate all American personnel from the annex to the Benghazi airport.
This article originally appeared in The New York Times.