The U.S. government insists it has the intelligence to prove it, but the public has yet to see a single piece of concrete evidence produced by U.S. intelligence — no satellite imagery, no transcripts of Syrian military communications — connecting the government of President Bashar Assad to the alleged chemical weapons attack last month that killed hundreds of people.

In its absence, Damascus and its ally Russia have aggressively pushed another scenario: that rebels carried out the Aug. 21 chemical attack. Neither has produced evidence for that case, either. That’s left more questions than answers as the U.S. threatens a possible military strike.

The early morning assault in a rebel-held Damascus suburb known as Ghouta was said to be the deadliest chemical weapons attack in Syria’s 2½-year civil war. Survivors’ accounts, photographs of many of the dead wrapped peacefully in white sheets and dozens of videos showing victims in spasms and gasping for breath shocked the world and moved President Barack Obama to call for action because the use of chemical weapons crossed the red line he had drawn a year earlier.