The unclassified assessment was released Friday as President Barack Obama mulled what action to take against President Bashar Assad, who has been locked in battle with rebels since March 2011.
It noted there was additional classified information backing up the conclusions.
"A large body of independent sources indicates that a chemical weapons attack took place in the Damascus suburbs on Aug. 21," the assessment reads. "In addition to U.S. intelligence information, there are accounts from international and Syrian medical personnel, videos, witness accounts, thousands of social media reports from at least 12 different locations in the Damascus area, journalist accounts and reports from highly credible non-governmental organizations."
The assessment indicates 1,429 people, including 426 children, died in the attack and about 3,600 people were treated at hospitals for symptoms.
"We assess with high confidence that the Syrian government carried out the chemical weapons attack against opposition elements in the Damascus suburbs on Aug. 21," the assessment says. "We assess that the scenario in which the opposition executed the attack on Aug. 21 is highly unlikely."
The assessment noted Syria possesses known stockpiles of mustard gas, sarin and VX, along with the munitions needed to deliver them. It alleges the weapons were used several times earlier in the year on a smaller scale.
U.S. Secretary of State John Kerry Friday called Assad a "murderer" and said the United States has unequivocal evidence the attack was launched by forces loyal to the Assad regime.