Why they are convinced it was the Syrians, and was sarin:

• The "immense" quantity of video and eyewitness evidence, as well as intelligence — all pointing toward a chemical weapons (CW) attack — is too massive and consistent to have been fabricated.
• Personnel affiliated with the CW program had previously been spotted at the base from which the attack was launched, and were there the day before the attack.
• Victims showed symptoms of sarin exposure, and the regime is the only actor in the area with access to sarin.

Why they are convinced Russia is lying:

• "Level of cooperation" between Russia and Syria and holes in Russian narrative make it "clear that the Russians are trying to cover up what happened here."
• Russia says the deadly gas was released after a bomb hit a munitions depot, but the munition the US has seen containing sarin landed in the middle of the street.
• "We do think that it's a question worth asking the Russians, about how is it possible that their forces were co-located with the Syrian forces that prepared and carried out the chemical weapons attack and did not have foreknowledge."

Why Syria would carry out such an attack:

• A rebel offensive in the area had threatened a key air base: "They were losing in a particularly important area and that's what drove it."
• "At that point the regime, we believe, determined that with its manpower spread quite thin... chemical weapons were necessary to make up for the manpower deficiency."
• They wanted to hit the civilian area to put pressure on rebels operating nearby.

On the potential for further attacks:

• "We take very seriously the possibility that Syria may have other agents elsewhere" and are looking into where the munitions may be and who is controlling them.
• More than 200 allegations of CW use since 2013, "we assess that many of those are credible."