The White House on Tuesday released an intelligence assessment saying it is “confident” that Syrian President Bashar Assad used sarin gas in a chemical attack on his people earlier this month.

The four-page, declassified report is meant to push back on what it called Russian efforts to spread “disinformation” about the origins of the attack in order to cover up the involvement of Assad, Moscow’s closest ally in the region.

“It’s clear the Russians are trying to cover up what happened there," a senior White House official said.

The White House did not say whether it believes Russia had prior knowledge of the chemical strike, yet questioned how its government could have not known.

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“How is it possible that their forces were co-located with the Syrian forces that planned, prepared and carried out this chemical weapons attack at the same installation and did not have foreknowledge?” another senior White House official told reporters.

The April 4 attack in a rebel-held area of Syria killed close to 90 people, including dozens of children, and prompted President Trump to retaliate by launching cruise missies at the base where the attack originated.

The report offered up evidence to rebut claims from Russians and others that deaths were the result of a government strike on an opposition weapons depot containing chemical weapons, that the chemical weapons attack was carried out by a terrorist group, such as the Islamic State in Iraq and Syria, or that the attack was faked.

It says “physiological” evidence collected from victims showed “symptoms consistent with sarin exposure,” including frothing at the mouth and twitching. Responders also suffered symptoms from sarin exposure.

The White House says evidence of the chemical attack began to surface on social media a 6:55 a.m., while Moscow said the attack occurred around midday.

“The absolute mass of data we have … it’s just too massive for really any intelligence organization to fabricate in that short of period of time,” another White House official said. “That’s just not a feasible explanation.”

White House officials said that the Syrian regime had motivation to attack the rebel-held area with chemical weapons, using them as a force multiplier because of its lack of manpower in the specific region.

The incident has sparked a major diplomatic confrontation between the U.S. and Russia. Secretary of State Rex Tillerson is in Moscow Tuesday but will not meet with President Vladimir Putin.

Putin, in turn, has suggested the U.S. plans to stage a chemical weapons attack in Syria and blame it on Assad.