U.S. intelligence officials said Friday that no evidence has surfaced to indicate that the Sept. 11 assault on a U.S. diplomatic outpost in Libya was planned in advance, a conclusion that suggests the attack was spontaneous even if it involved militants with ties to al-Qaeda.

The description represents the latest shift in the U.S. government’s evolving account of an attack that claimed the life of the U.S. ambassador to Libya, as well as three other U.S. citizens, and has become entangled in the politics of the presidential campaign.

“There isn’t any intelligence that the attackers pre-planned their assault days or weeks in advance,” a U.S. intelligence official said. “The bulk of available information supports the early assessment that the attackers launched their assault opportunistically after they learned about the violence at the U.S. Embassy in Cairo.”

That emerging consensus among analysts at the CIA and other agencies could lend new support to the Obama administration, which has struggled to fend off Republican allegations that it has been reluctant to admit that the attack in Benghazi was an act of terrorism.

Much of that Republican criticism has focused on U.S. Ambassador to the United Nations Susan E. Rice, who appeared on television talk shows days after the attack and attributed it to violent protesters angered by an anti-Muslim YouTube video. The latest assessment indicates that the timing of the attack in Benghazi was triggered by protests, but also supports subsequent accounts by Obama administration officials describing the siege as a terrorist assault.

As a result, the information provided by U.S. intelligence officials on Friday is unlikely to end the controversy surrounding a narrative that has already shifted several times.

House Republicans expanded their scrutiny of the administration Friday, pressing President Obama to address whether the White House played a role in decisions on security at the Benghazi mission.

“Your administration has not been straightforward with the American people in the aftermath of the attack,” House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.) and Rep. Jason Chaffetz (R-Utah) wrote to Obama.

Separately, Senate Homeland Security and Governmental Affairs Committee Chairman Joseph I. Lieberman (I-Conn) and Sen. Susan Collins (R-Maine), the panel’s top Republican, requested documents and a classified briefing on the attack in letters to Secretary of State Hillary Rodham Clinton, Defense Secretary Leon E. Panetta and Director of National Intelligence James R. Clapper Jr.

U.S. officials have backed away from claims that protesters had gathered around the Benghazi mission before it was overrun. Instead, analysts now think that the siege involved militants who “may have aspired to attack the U.S. in Benghazi,” and mobilized after seeing protesters scale the walls of the embassy in Cairo to protest the controversial video.

The violence in Benghazi appears to have involved militants with ties to al-Qaeda in North Africa, but no evidence indicates that it was organized by al-Qaeda, or timed to coincide with the anniversary of the Sept. 11, 2001, terrorist attacks in the United States, officials said.

CIA officers filed a series of cables during the assault, but those amounted to tersely worded incident reports, and did not include assessments on what had triggered the attack, officials said.

U.S. officials also said there were no American surveillance drones over the compounds until the next morning, when evacuations were underway, meaning there was no aerial footage to show how the attack began.

More world news coverage:

- Car bombing in Beirut kills at least 8

- Pakistan to dress down as it heats up

- Europe agrees to banking regulator

- Read more headlines from around the world

Anne Gearan contributed to this report.