WASHINGTON — A summary of the Obama administration’s case that the Syrian regime is responsible for a chemical weapons attack Aug. 21 on the Damascus suburbs that it says killed 1,429 people:

WHY: The Syrian government has been frustrated as it tries to clear rebels out of several Damascus suburbs. The government considers chemical weapons a normal part of its military arsenal, and it has carried out such attacks in the 2½-year conflict before.

HOW: Troops from Syria’s military unit that handles chemical weapons, the Syrian Scientific Studies and Research Center, prepared such weapons in the three days prior to the attack. Another Syrian military unit was detected preparing gas masks in readiness for such an attack.

WHEN: On the morning of Aug. 21, at 2:30 a.m. local time, multiple streams of intelligence including satellite images show a barrage of rocket and artillery fire was launched from regime-controlled territory into the Damascus suburbs where the chemical weapons attacks reportedly occurred, approximately 90 minutes before reports of the attack started appearing in social media from affected areas.

VICTIMS: Three Damascus area hospitals were flooded in a three-hour period by approximately 3,600 patients showing symptoms consistent with nerve agent exposure, according to an international humanitarian organization.

KILLED: The chemical attack killed 1,429 people, including at least 426 children, according to U.S. intelligence and accounts from international and Syrian medical personnel, and nongovernmental organizations, and hundreds videos scoured from social media of the aftermath, posted from 12 different areas. The videos show large numbers of victims showing signs “consistent with, but not unique to, nerve agent exposure,” including unconsciousness, foaming from the nose and mouth, constricted pupils, rapid heartbeat, and difficulty breathing. The videos also show large numbers of dead with no visible injuries, which points to death by chemical weapons, not small arms, high-explosive munitions or blister agents.

GOVERNMENT LINKS: U.S. intelligence intercepted communications of a “senior official intimately familiar with the offensive” who confirmed that chemical weapons were used by the Syrian government, and was concerned that the U.N. inspectors might find evidence of the attack. Also, the U.S. has intelligence that Syrian chemical weapons personnel were directed to “cease operations” on the afternoon of Aug. 21, several hours after the attack. At the same time, the Syrian military intensified its conventional artillery barrage of the area, upping it to four times above the amount of artillery and rocket fired in the previous 10 days, and the U.N. inspectors were shot at as they tried to inspect the areas of where the suspected chemical attacks occurred.

WHO ORDERED IT: Not important, the administration says. While officials would not discuss whether they had proof that Syrian President Bashar Assad himself ordered the attack, they said they believe the program is under his command and control, and therefore is his responsibility.

WHO CONTROLS CHEMICAL WEAPONS: Administration officials have insisted they believe the supplies remain in Syrian government control, although other U.S. officials say some of those supplies may have been lost to rebel groups who overtook military storage areas in the last six months.

NO REBELS SUSPECTED: U.S. intelligence officials say that is “highly unlikely” that Syrian rebels who would like to draw the U.S. into the war to draw on its firepower would be behind the chemical attack. Officials say the opposition has not used chemical weapons in the past