Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin. This assessment is based in part on physiological samples. Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions. We do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime.