A three-page report released Thursday by the British government said there was "a limited but growing body of intelligence" blaming the Syrian government for the attacks. And though the British were not sure why Assad would have carried out such an attack, the report said there was "no credible intelligence" that the rebels had obtained or used chemical weapons.
Quizzed by lawmakers in Britain's House of Commons, Prime Minister David Cameron gave various descriptions for his level of certainty to Assad's responsibility, ranging from "beyond doubt" to being "as certain as possible."
"We have a regime that has used chemical weapons on 14 occasions, that is most likely responsible for this large-scale attack, that if nothing is done it will conclude that it can use these weapons again and again and on a larger scale and with impunity," he said.
Like the British report, the yet-to-be-released U.S. report assesses with "high confidence" that the Syrian government was responsible for the attacks that hit suburbs east and west of Damascus, according to a senior U.S. official who read the report.
The humanitarian group Doctors Without Borders has said the strikes killed 355 people.
The complicated intelligence picture raises questions about the White House's approach to the attack on a rebel-held Damascus suburb. Administration officials said Wednesday that neither the U.N. Security Council nor allies' concerns would affect their plans.
Intelligence officials say they could not pinpoint the exact locations of Assad's supplies of chemical weapons, and Assad could have moved them in recent days. That lack of certainty means a possible series of U.S. cruise missile strikes aimed at crippling Assad's military infrastructure could hit newly hidden supplies of chemical weapons, accidentally triggering a deadly chemical attack.
Over the past six months, with shifting front lines in the 2-year-old civil war and sketchy intelligence coming out of Syria, U.S. and allied spies have lost track of who controls some of the country's chemical weapons supplies, according to one senior U.S. intelligence official and three other U.S. officials briefed on the intelligence shared by the White House.
All spoke on condition of anonymity because they were not authorized to discuss the Syrian issue publicly.
U.S. satellites have captured images of Syrian troops moving trucks into weapons storage areas and removing materials, but U.S. analysts have not been able to track what was moved or, in some cases, where it was relocated.
In addition, an intercept of Syrian military officials discussing the strike was among low-level staff, with no direct evidence tying the attack back to an Assad insider or even a senior Syrian commander, the officials said.
So while Secretary of State John Kerry said Monday that it was "undeniable" a chemical weapons attack had occurred, and that it was carried out by the Syrian military, U.S. intelligence officials are not so certain that the suspected chemical attack was carried out on Assad's orders, or even completely sure it was carried out by government forces, the officials said.
Ideally, the White House seeks intelligence that links the attack directly to Assad or someone in his inner circle to rule out the possibility that a rogue element of the military decided to use chemical weapons without Assad's authorization.
The U.S. has devoted only a few hundred operatives, between intelligence officers and soldiers, to the Syrian mission, with CIA and Pentagon resources already stretched by the counterterrorism missions in Africa and the Arabian Peninsula, as well as the continuing missions in Afghanistan and Pakistan, officials said.
The quest for added intelligence to support the White House's case for a strike against Assad's military infrastructure delayed the release of the U.S. intelligence community's report, which had been expected Tuesday.
The CIA, the Pentagon and the Office of the Director of National Intelligence declined to comment, and the White House did not respond to requests for comment.
Still, many U.S. lawmakers believe there is reasonable certainty Assad's government was responsible and are pressing the White House to go ahead with an armed response.
Others, both Democrats and Republicans, have expressed serious concern with the expected military strike.
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Associated Press writers Kimberly Dozier, Matt Apuzzo, Bradley Klapper, Julie Pace and Lara Jakes contributed to this report.
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Follow Dozier on Twitter: http://twitter.com/kimberlydozier
and Apuzzo at http://twitter.com/mattapuzzo
