Two U.S. officials told NBC News on Thursday afternoon that two different chemical weapons – chlorine gas and an unidentified nerve agent – were found in blood and urine samples from victims of Saturday’s suspected WMD attack in Syria.

The Syrian government has used chlorine on previous occasions, but medical personnel treating victims of the attack reported seeing the symptoms of a nerve agent like sarin as well. The number of people who died and the speed of their demise also argues in favor of a nerve agent mixed with the chlorine, possibly in the hope of using chlorine to conceal the deadlier ingredient.

NBC’s sources were said to be “confident” of the intel but “not 100 percent sure.” That seems an odd way of putting it, given that the samples are supposedly in American hands and have been tested by U.S. technicians.

A common concern voiced about this, and most other, alleged chemical attack in Syria is that information from the site is not completely reliable. The Syrian regime makes it difficult for international inspectors to conduct first-hand research and routinely blames rebel forces for either perpetrating or faking chemical attacks. Rebel groups are not always reliable eyewitnesses, to put it delicately. Authentic samples tested directly by U.S. and allied experts would be the gold standard for solid evidence of a WMD (Weapon of Mass Destruction) strike.

The officials who spoke to NBC also said that the U.S. and other countries have intelligence pinning responsibility for the attack on the Syrian government, including photographic images.

French President Emmanuel Macron also declared on Thursday that he has “proof” the Syrian government attacked the town of Douma with chemical weapons. Macron specified that “at least chlorine” was used by the Assad regime.

The Organization for the Prohibition of Chemical Weapons (OPCW) is sending inspectors to Syria, but they are not scheduled to reach Douma until Saturday at the earliest.

“I believe there was a chemical attack. We’re looking for the actual evidence,” Secretary of Defense James Mattis told the House Armed Services Committee on Thursday.

Mattis referenced the OPCW mission to Syria and complained about the “challenges we face where Russia has six times in the U.N. rejected and made certain that we could not get investigators in.” He cautioned that even if inspectors confirmed chemical weapons were used in Douma, “we will not know who did it.”