BEIRUT (AP) — The U.S. government insists it has the intelligence to prove Syrian government forces carried out a chemical weapons attack last month in a rebel-held suburb of Damascus, but so far it has not made public any concrete evidence.

The Obama administration says its assessment is based mainly on satellite and signal intelligence. But multiple requests to view the satellite imagery have been denied, as has a request by The Associated Press to see a transcript of communications allegedly ordering Syrian military personnel to prepare for a chemical weapons attack by readying gas masks.

Open-source evidence does provide clues about the attack, including videos of the rockets that analysts believe were likely used. U.S. officials yesterday released a compilation of videos showing victims, including children, exhibiting what appear to be symptoms of nerve gas poisoning.

Some experts say the size of the strike and the amount of toxic chemicals that appear to have been delivered make it doubtful that the rebels could have carried it out, a scenario that the Assad government and its ally Russia have aggressively pushed. But what's missing from the public record is direct proof, rather than circumstantial evidence, tying the attack to the regime.