WASHINGTON (AP) — U.S. Secretary of Defense James Mattis confirmed that the U.S. government has no evidence that the Syrian government used sarin gas on its people— a claim that was used by the White House as justification for an April 2017 launch of 59 Tomahawk cruise missiles at the Al Shayrat airfield in Syria.

On Friday, Mattis said that reports of chemical weapon use by the Syrian government have come from aid groups and others, but that the U.S. doesn’t have any evidence to support these assertions.

“We have other reports from the battlefield from people who claim it’s been used,” Mattis told reporters at the Pentagon. “We do not have evidence of it.”

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“We’re looking for evidence of it, since clearly we are dealing with the Assad regime that has used denial and deceit to hide their outlaw actions,” Mattis continued. “We’re even more concerned about the possibility of sarin use.”

Mattis explained that he was not refuting the third-party reports of chemical weapons used by the Syrian government led by President Bashar Assad. Assad has steadfastly denied that his government has used chemical weapons throughout the conflict.

In 2013, UN investigator Carla Del Ponte made note that Syrian rebels, not the Assad regime, used chemical weapons in the two-year civil war, contrary to assessments made by American officials.

According to a report by The Times of Israel:

“Carla Del Ponte, head of the independent UN commission investigating reports of chemical weapons use in Syria, told a Swiss-Italian television station that UN investigators gleaned testimony from victims of Syria’s civil war and medical staff which indicated that rebel forces used sarin gas — a deadly nerve agent.

‘Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals and, according to their report of last week which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated,’ Del Ponte said in the interview, translated by Reuters.

‘This was use on the part of the opposition, the rebels, not by the government authorities,’ she added.”

During his comments on Friday, Mattis referred to the April 2017 cruise missile strikes on a Syrian airbase, noting that the Syrian government would “be ill-advised to go back to violating” the chemical weapons prohibition.

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In addition to the UN investigation, one of the foremost academic experts in the field of missile fired chemical weapons, Theodore Postol, Professor of Science, Technology and National Security Policy at the Massachusetts Institute of Technology (MIT), came forward in a series of reports to note his opposition to the official Trump administration’s narrative in regards to the Khan Sheikhoun nerve agent attack in Syria, blamed on the Assad government, which precipitated the cruise missile strikes by the U.S., according to a report in the International Business Times. According to Postol, the Syrian gas attack was not carried out by the Syrian government.

In one of his reports, Postol concluded that the US government’s report does not provide any “concrete” evidence that Assad was responsible, adding it was more likely that the attack was perpetrated by players on the ground.

Postol wrote in his report:

“I have reviewed the [White House’s] document carefully, and I believe it can be shown, without doubt, that the document does not provide any evidence whatsoever that the US government has concrete knowledge that the government of Syria was the source of the chemical attack in Khan Sheikhoun, Syria at roughly 6am to 7am on 4 April, 2017.

In fact, a main piece of evidence that is cited in the document point to an attack that was executed by individuals on the ground, not from an aircraft, on the morning of 4 April.

This conclusion is based on an assumption made by the White House when it cited the source of the sarin release and the photographs of that source. My own assessment is that the source was very likely tampered with or staged, so no serious conclusion could be made from the photographs cited by the White House.”

Postol noted that he has “unambiguous evidence that the White House Intelligence Report (WHR) of April 11, 2017 contains false and misleading claims that could not possibly have been accepted in any professional review by impartial intelligence experts.”

Postol called for an independent investigation into the decision to launch cruise missile strikes in Syria, concluding:

“It is now obvious that this incident produced by the WHR, while just as serious in terms of the dangers it created for US security, was a clumsy and outright fabrication of a report that was certainly not supported by the intelligence community.

In this case, the president, supported by his staff, made a decision to launch 59 cruise missiles at a Syrian air base. This action was accompanied by serious risks of creating a confrontation with Russia, and also undermining cooperative efforts to win the war against the Islamic State.”