WASHINGTON — Three days before the rockets fell in eastern Damascus, a team of Syrian specialists gathered in the northern suburb of Adra for a task that U.S. officials say had become routine in the third year of the country's civil conflict: filling warheads with deadly chemicals to kill Syrian rebels.
The preparations, as described by U.S. intelligence analysts, continued from Aug. 18 until just after midnight on Aug. 21, when the projectiles were loaded into rocket launchers behind the government's defensive lines. Then, at 2:30 a.m., a half-dozen densely populated neighborhoods were jolted awake by a series of explosions, followed by an oozing blanket of suffocating gas.
Unknown to Syrian officials, U.S. spy agencies recorded each step in the alleged attack, from the extensive preparations to the launching of rockets to the after-action assessments by Syrian officials. Those records and intercepts would become the core of the Obama administration's evidentiary case linking the Syrian government to the use of outlawed toxins to kill 1,429 civilians, including at least 426 children.
Pulling back the curtain on some of United States' most sensitive collection efforts, the Obama administration released on Friday its long-awaited intelligence assessment of the Aug. 21 event, explaining in rare detail the basis for its claim that Syria was behind the release of deadly gas, the grisly effects of which have been documented in more than 100 amateur videos.
The four-page assessment and accompanying map revealed for the first time how communications intercepts and satellite imagery picked up key decisions and actions on the ground.
While unusually detailed, the assessment did not include photographs, recordings or other hard evidence to support its claims. Nor did it offer proof to back up the administration's assertion that top-ranking Syrian officials — possibly including President Bashar Assad — were complicit in the attack.
"There is additional intelligence that remains classified because of sources and methods concerns," the report said. "That is being provided to Congress and international partners."
The material, prepared by senior intelligence officials, was said to reflect the judgments of intelligence agencies involved in gathering information on the Syrian conflict. It asserts with "high confidence" that the Assad government launched a chemical weapons attack on eastern Damascus, using what it said was "nerve agent," a class of chemical munitions that includes sarin. Such weapons are banned under an international treaty that Syria signed but never ratified.
Echoing the findings of a British intelligence assessment a day earlier, the report linked the Assad government to "multiple" chemical weapons attacks in the past year, including a small-scale attack in the same part of Damascus on Aug. 21.
It suggested that a relatively controlled use of chemicals had in recent months become part of the normal military strategy whenever government forces were unable to push back rebel offensives.
"The Syrian regime has used chemical weapons over the last year primarily to gain the upper hand or break a stalemate in areas where it had struggled to seize and hold strategically valuable territory," it said. "We assess that the regime's frustration with its inability to secure large portions of Damascus may have contributed to its decision to use chemical weapons on Aug. 21."
Indeed, in the week before the attack, government troops had repeatedly failed to dislodge rebels from dozens of villages east of Damascus, despite the use of aircraft, helicopters and heavy artillery barrages. The fighting appeared to have stalled when, on Aug. 18, U.S. intelligence agencies observed a team of chemical weapons specialists being activated in Adra, a northern suburb "near an area that the regime uses to mix chemical weapons, including sarin," the document states.
On Aug. 21, satellites and other surveillance aircraft picked up the flashes of rockets and artillery shells being fired from government positions toward rebel-held and contested villages east and south of the city. The first reports of a gas attack appeared on social media shortly after the projectiles landed.
As the scale of the civilian casualties was becoming clear, U.S. monitors made another key intercept, this time a message to a Syrian chemical weapons team that was "directed to cease operations," the U.S. assessment said. Later that day, other Syrian troops began an intensified barrage on the same neighborhood using conventional artillery in an apparent effort to cover up the evidence.