The White House report reads:
On August 21, a Syrian regime element prepared for a chemical weapons attack in the Damascus area, including through the utilization of gas masks. Our intelligence sources in the Damascus area did not detect any indications in the days prior to the attack that opposition affiliates were planning to use chemical weapons.
Local social media reports of a chemical attack in the Damascus suburbs began at 2:30 a.m. local time on August 21. We have identified one hundred videos attributed to the attack, many of which show large numbers of bodies exhibiting physical signs consistent with, but not unique to, nerve agent exposure. The reported symptoms of victims included unconsciousness, foaming from the nose and mouth, constricted pupils, rapid heartbeat, and difficulty breathing. Several of the videos show what appear to be numerous fatalities with no visible injuries, which is consistent with death from chemical weapons, and inconsistent with death from small-arms, high-explosive munitions or blister agents. At least 12 locations are portrayed in the publicly available videos, and a sampling of those videos confirmed that some were shot at the general times and locations described in the footage.
We assess the Syrian opposition does not have the capability to fabricate all of the videos,*physical symptoms verified by medical personnel and NGOs, and other information associated with this chemical attack.*We have a body of information, including past Syrian practice, that leads us to conclude that regime officials were witting of and directed the attack on August 21. We intercepted communications involving a senior official intimately familiar with the offensive who confirmed that chemical weapons were used by the regime on August 21
Critics are asking why the hundred of videos of the attack haven't been presented to the public (though there remain ethical arguments against that) and why there remains a difference between indicating evidence ("We asses," "Our intelligence sources say," "We have identified") and actually revealing evidentiary sources. At the end of the report, it's noted that much information must remain top-secret because of the nature of the project.
After all, we've become accustomed to detailed information, courtesy of Edward Snowden and Bradley Manning. The White House's document seems vague in that light, but declassifying a document of that nature is a rare move for any administration. Certainly the public didn't have any such "insider knowledge" before the 2003 invasion of Iraq.