NBC reported on Thursday that two U.S. officials said that blood and urine samples from victims of the chemical attack in Syria over the weekend tested positive for a nerve agent and chlorine gas.

The backdrop: A few hours earlier, Defense Secretary Jim Mattis said that the administration was "looking for the actual evidence" to support a chemical attack, which affected 500 people. Mattis also said there would be a National Security Council meeting at the White House on Thursday to discuss options for response in Syria.