The White House ideally wants intelligence that links the attack directly to Assad or someone in his inner circle, to rule out the possibility that a rogue element of the military decided to use chemical weapons without Assad's authorization. That quest for added intelligence has delayed the release of the report laying out evidence against Assad. The report was promised earlier this week by administration officials.

The CIA and the Pentagon have been working to gather more human intelligence tying Assad to the attack, relying on the intelligence services of Jordan, Saudi Arabia and Israel, the officials said

Both the CIA and the Defence Intelligence Agency have their own human sources — the rebel commanders and others who cross the border to brief CIA and defence intelligence officers at training camps in Jordan and Turkey. But their operation is much smaller than some of the other intelligence services, and it takes longer for their contacts to make their way overland.

Obama said he was not seeking a lengthy, open-ended conflict in Syria, indicating that any U.S. response would be limited in scope. But he argued that Syria's use of chemical weapons not only violated international norms, but threatened "America's core self-interest."

"We do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable," he said.

http://www.cbc.ca/news/world/story/2013/08/29/syria-us-response-studied.html