"Satellite detections corroborate that attacks from a regime-controlled area struck neighborhoods where the chemical attacks reportedly occurred," a declassified White House report says. "... The lack of flight activity or missile launches also leads us to conclude that the regime used rockets in the attack."
The White House released a map, but no satellite images.
The report also cites "multiple streams of intelligence," without giving specifics.
"In the three days prior to the attack, we collected streams of human, signals and geospatial intelligence that reveal regime activities that we assess were associated with preparations for a chemical weapons attack," the U.S. report says.
"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."