"Syrian chemical weapons personnel were operating in the Damascus suburb of 'Adra... near an area that the regime uses to mix chemical weapons, including sarin. On August 21, a Syrian regime element prepared for a chemical weapons attack in the Damascus area, including through the utilization of gas masks."
"We have a body of information, including past Syrian practice, that leads us to conclude that regime officials were witting of and directed the attack," the report says. "... We intercepted communications involving a senior official intimately familiar with the offensive who confirmed that chemical weapons were used by the regime on August 21 and was concerned with the U.N. inspectors obtaining evidence."
Intelligence shows Syrian chemical weapons personnel were told to cease operations in the afternoon of August 21 and that the regime then "intensified the artillery barrage" in the area, the report says.
The material remains classified.
U.S.: Opposition doesn't have 'the capacity'
U.S., British, and French intelligence reports all agree that the opposition couldn't have pulled off such an attack.
"We are certain that none of the opposition has the weapons or capacity to effect a strike of this scale, particularly from the heart of regime territory," Kerry told lawmakers.
The White House report points to Syria's known stockpiles of chemical agents. And it says the United States assesses "with high confidence that the Syrian regime has used chemical weapons on a small scale against the opposition multiple times in the last year, including in the Damascus suburbs. This assessment is based on multiple streams of information including reporting of Syrian officials planning and executing chemical weapons attacks and laboratory analysis of physiological samples obtained from a number of individuals, which revealed exposure to sarin.
"We assess that the opposition has not used chemical weapons."
In May, a U.N. official said there were strong suspicions that Syrian rebel forces had used sarin gas. But the findings were not conclusive, the U.N. Independent International Commission of Inquiry for Syria said at the time, and the opposition Syrian Coalition condemned any use of chemical weapons. The U.S. State Department said at the time it had no evidence suggesting rebels had used chemical weapons.
Russia, a Syrian ally, says its investigation of a March attack in Aleppo, which apparently involved chemical weapons, found that the charge used was homemade and similar to projectiles produced by the group Bashaar al-Nasr, part of the opposition Syrian Islamic Liberation Front. Sarin was discovered in samples from the scene, the foreign ministry said.
Assad's motive unclear
Some experts on the region question why al-Assad would have ordered the attack.
"Al Assad has no credible motivation to use these weapons at this stage, and in this phase of the conflict. He is not losing," writes Ed Husain of the Council on Foreign Relations in a CNN Opinion column. He pointed out that some suggest the al Qaeda-affiliated al-Nusra Front or other opposition elements may have carried out the attack to bait America into the conflict.
William Polk, who served U.S. administrations during the Cuban Missile Crisis and and 1967 Middle East War, writes in The Atlantic, "I do not see what Assad could have gained from this gas attack."
Analyst: 'No way in hell' U.S. can back up death toll
Questions about the purported death toll in last month's attack also raises questions about the solidity of the information the U.S. is using.
A preliminary assessment "determined that 1,429 people were killed in the chemical weapons attack, including at least 426 children," the U.S. report says.
"Secretary Kerry seems to have been sandbagged into using an absurdly over-precise number," says Anthony Cordesman, former director of intelligence assessment at the U.S. Defense Department.
Now with the Center for Strategic and International Studies, Cordesman writes on the CSIS website, "Put simply, there is no way in hell the U.S. intelligence community could credibly have made an estimate this exact."
The methodology used to come up with the toll remains classified.
Rebel leaders have said more than 1,300 people were killed. Britain's Joint Intelligence Organization says at least 350 people were killed. A French report says several sources estimated at least 355 deaths, while others estimate 1,500.
U.N. won't place blame
The United Nations is calling on world leaders not to take action until the results of the U.N. probe are in. But it's unclear how soon that may be.
And the U.N. team's mandate was only to determine whether chemical weapons were used -- not by whom.