A Syrian chemical weapons attack killed 1,429 Syrian civilians, including 426 children, an unclassified report by US intelligence agencies concluded on Friday.
US President Barack Obama is using the report to make the case for retaliation against the Syrian government.
The report said US authorities have a high degree of confidence that the Syrian government of Bashar Assad was responsible for the attack. It stated that this is the strongest position the US intelligence community that vetted the report can take short of confirmation.
A central piece of intelligence included a communication that was intercepted from "a senior official intimately familiar with the attack," who confirmed that chemical weapons had been used by the government on August 21 and was "concerned with the UN inspectors obtaining evidence" about it.
The intercepted phone call between Assad regime officials trying to cover up the attack "speaks to a level of concern on their part vis-a-vis potential detection and attribution," the officials said. "We do assess that [Assad] is the decision maker and he directs employment... the overall program is firmly under his control."
The White House released a map detailing the attack to accompany the report.
The report said a nerve agent was used in the attack, which took place in the Damascus suburbs and was aimed at ridding the area of those trying to topple the Assad government.
"What you're struck by when you look at this is just how brought the attack was," the officials said. "We assess that the regime considers chemical weapons in their portfolio of military use."
In a press conference at the State Department, US Secretary of State John Kerry said that according to the facts known to the US intelligence community, the Assad regime - which has the largest chemical weapons arsenal in the Middle East - has used its chemical stockpile multiple times this year on civilians.
Three days before the attack, regime chemical weapons personnel were on the ground making preparations, and regime elements were told to prepare for the attack with "specific instructions" to wear gas masks and taking other precautions associated with chemical weapons, Kerry said.
The US also knows where the chemical rockets were launched from, at what time they were launched and where they landed, he added.
Ninety minutes after the attack "all hell broke loose on social media," Kerry said, saying 11 different sites reported on victims with symptoms associated with chemical weapons, such as difficulty breathing, twitching, foaming at the mouth, unconsciousness and death.
There were no signs of physical injury on the victims' bodies, like shrapnel wounds, cuts, or gunshot wounds, and the white linen wrapped around bodies of those killed did not have one drop of blood on it, Kerry said.
The secretary of state stressed the victims were ordinary citizens. Some of the victims, he said, were first responders - doctors and nurses on the scene who were affected by the chemical agents.
The death toll given by the report was the first precise number and far larger than previously estimated. A senior administration official who briefed reporters said the number could rise.
The report said the conclusion was based on human, signals and satellite intelligence as well as a significant body of public material, such as amateur videos.
The report said three hospitals in the Damascus area received some 3,600 patients showing symptoms consistent with nerve agents in less than three hours on the morning of August 21.
Rejecting claims by the Syrian government that the Syrian opposition conducted the attack, the report said the rebels had no capability to fabricate all of the videos and the physical symptoms verified by medical personnel.