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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 what one official called an “indiscriminate, unspeakable horror” – the use of outlawed toxins to kill nearly 1,500 civilians, including at least 426 children.

Pulling back the curtain on some of United States’ most sensitive intelligence collection efforts, the Obama administration on Friday released 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.

Meanwhile, U.N. chemical weapons experts have left Syria and crossed into neighboring Lebanon early Saturday.

The team on Friday carried out a fourth and final day of inspection as they sought to determine precisely what happened in the Aug. 21 alleged chemical weapons attack near Damascus.

The team took samples from victims for examination in laboratories in Europe.

The four-page U.S. intelligence assessment and accompanying map revealed for the first time how communications intercepts and satellite imagery picked up key decisions and actions on the ground.

In choosing to release the document, White House officials anticipated the likely comparisons to the famously inaccurate intelligence reports from a decade ago that claimed that Iraqi President Saddam Hussein was actively pursuing nuclear, chemical and biological weapons. Secretary of State John Kerry, in his remarks on the release of the intelligence assessment, said White House officials were “more than mindful of the Iraq experience.”

“We will not repeat that moment,” Kerry said. “Accordingly, we have taken unprecedented steps to declassify and make facts available to people who can judge for themselves.”

The document proposes a possible motive for the attack – a desperate effort to push back rebels from several areas in the capital’s densely packed eastern suburbs – and suggests that the high civilian death toll surprised and panicked senior Syrian officials, who called off the attack and then tried to cover it up.

“We intercepted communications involving a senior official intimately familiar with the offensive,” it says, “who confirmed that chemical weapons were used by the regime on Aug. 21 and was concerned with the U.N. inspectors obtaining evidence.”

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.”

Among the surprises in the report was the U.S. estimate for the dead and wounded. The new figure, 1,429, was nearly four times higher than a British casualty estimate released Thursday.

The material, prepared by senior intelligence officials, was said to reflect the judgments of the CIA, National Security Agency and other intelligence agencies. The report 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. It suggested that a relatively controlled used of chemicals had in recent months become part of the normal military strategy whenever government forces were unable to push back rebel offensives or break through defensive fortifications.

“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.”

In the week before the attack, government troops had been repeatedly frustrated in their efforts 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. The ordnance team was attached to Syria’s Scientific Studies and Research Center, a military-run institute linked to Syria’s chemical and nuclear weapons programs.

On Aug. 21, regime forces were observed making further preparations for a chemical attack, “including through the utilization of gas masks,” according to the intelligence summary. Soon afterward, 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 Web sites very shortly after the projectiles landed.