WASHINGTON — The CIA rushed security operatives to an American diplomatic compound in Libya within 25 minutes after it had come under attack and played a more central role in the effort to fend off a nightlong siege than has been acknowledged publicly, U.S. intelligence officials said Thursday.

The agency mobilized the evacuation effort, took control of an unarmed U.S. military drone to map possible escape routes, dispatched an emergency security team from Tripoli, the capital, and chartered aircraft that ultimately carried surviving American personnel to safety, U.S. officials said.

The account provided by senior U.S. intelligence officials offers the most detailed chronology yet of the Sept. 11 assault that killed the U.S. ambassador to Libya and three other Americans. The attack has become a flash point in the U.S. presidential campaign.

The decision to give a comprehensive account of the attack five days before the election is likely to be regarded with suspicion, particularly among Republicans who have accused the Obama administration of misleading the public by initially describing the assault as a spontaneous eruption that began as a protest of an anti-Islamic video.

U.S. officials said they decided to offer a detailed account of the CIA’s role to rebut media reports that have suggested that agency leaders delayed sending help to State Department officials seeking to fend off a heavily armed mob.

Instead, U.S. intelligence officials insisted that CIA operatives in Benghazi and Tripoli made decisions rapidly throughout the assault with no interference from Washington.

“There was no second-guessing those decisions being made on the ground, by people at every U.S. organization that could play a role in assisting those in danger,” a senior U.S. intelligence official said in a prepared statement that summarized the chronology of the attack and was made available to news organizations.

The information does not address the main source of political controversy surrounding the siege: the shifting assessments offered by Obama administration officials over whether the assault was a protest that turned violent or a planned terrorist attack.

But officials reiterated that the initial intelligence was fragmentary and often contradictory. They said talking points for members of Congress and senior administration officials did not discuss possible links between the attackers and al-Qaida because the information was classified.

“It wasn’t until after the points were used in public that people reconciled contradictory information and assessed there probably wasn’t a protest around the time of the attack,” the senior U.S. intelligence official said.

The briefing and material provided Thursday focused on the hour-by-hour developments in Benghazi. Among the disclosures is that the CIA station chief in Tripoli sent an emergency security force, with about a half-dozen agency operatives as well as two U.S. military personnel, to Benghazi aboard a hastily chartered aircraft while the attack was underway.

The team arrived after midnight and attempted to organize an effort to make its way to a hospital where U.S. Ambassador J. Christopher Stevens had been taken and was thought to be alive.

But the team was held up by a combination of the time required to secure transportation and arms from U.S.-allied Libyan militias, new reports that the ambassador was probably already dead and uncertainty about the security situation at the hospital.

The annex in Benghazi to which U.S. diplomatic personnel were evacuated was a CIA base that the agency had established as its first stronghold in Libya before autocrat Moammar Gadhafi was overthrown late last year.

U.S. officials said the CIA base learned of the assault on the nearby diplomatic compound in a desperate phone call about 9:40 p.m.

CIA security operatives assembled their gear and lined up vehicles even while agency officials sought, without success, to enlist Libyan militias that had been hired to provide security for the diplomatic outpost in Benghazi.

“Over the next 25 minutes, team members approach the compound, attempt to secure heavy weapons” from Libyans encountered along the way and “make their way onto the compound itself in the face of enemy fire,” the senior U.S. intelligence official said.

Shortly after 11 p.m., an unarmed Predator drone diverted from another mission arrived over Benghazi and began providing video surveillance.

The CIA operatives appear to have been part of a broader group of U.S. security personnel and Libyan guards who made several attempts to fight their way into a structure known as “Villa C,” which served as the safe house and the VIP residence for the mission, and where Stevens had taken cover. Each time, the would-be rescuers were forced to retreat from heavy smoke and flames that had engulfed the structure.

By 11:30 p.m., “all U.S. personnel, except for the missing U.S. ambassador, depart the mission,” the U.S. intelligence official said. “The exiting vehicles come under fire.”

By then, attackers had also descended on the CIA compound, about a mile from the diplomatic facility. The “annex,” as the CIA base was known in internal documents, continued to come under small-arms and rocket fire sporadically over the next 90 minutes.

Then, about 1 a.m., the siege went suddenly quiet, a pause that would last until near daybreak, apparently leading CIA and State Department officials to think that the danger had passed.

In the “pre-dawn time frame, that team at the airport finally manages to secure transportation and armed escort and — having learned that the ambassador was almost certainly dead — heads to the annex to assist with the evacuation,” the official said.

The team arrived, accompanied by Libyan security elements, at 5:15 a.m., “just before the mortar rounds begin to hit the annex,” the official said. Other accounts have suggested that multiple mortars were aimed at the site, initially missing their target before striking the roof, where guards had taken position and were returning fire.

Two CIA contractors, both former Navy SEALs, were killed: Tyrone Woods, a security officer based in Benghazi, and Glen Doherty, who was part of the team rushed by air from Tripoli.

That final spasm of violence lasted 11 minutes, officials said. “Less than an hour later, a heavily-armed Libyan military unit arrived to help evacuate the compound of all U.S. personnel,” the senior U.S. intelligence official said.

The account, though the most comprehensive to date, still leaves a number of questions unresolved. Among them are why the single Predator overhead was unarmed and whether a model equipped with Hellfire missiles might have been able to strike the mortar launching site used by the attackers or otherwise disrupt the assault.

Another question is why “a heavily-armed Libyan military unit” was able to arrive at the annex around sunrise but was not available earlier despite presumably frantic U.S. calls.