A Republican congressman is skeptical of the Trump administration's conclusion that Syrian President Bashar al-Assad carried out a chemical attack against his own people.

Rep. Thomas Massie (R-KY) told Tucker Carlson that he remains unconvinced that Assad was behind the attack, calling on the White House to put forth more evidence.

"Truth is the first casualty of war," he said, adding that the U.S., Russia and Syria will all have their own versions of what happened.

"And then there's gonna be the truth in there. We haven't been provided the details," he added.

Earlier this week, Defense Secretary James Mattis said "there is no doubt" that Assad’s regime is responsible for the chemical attack.

"I have personally reviewed the intelligence and there is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself," he said.

In recent days, Assad and Russian Foreign Minister Sergey Lavrov have claimed the attack was staged by the U.S. as pretext for last week's missile attack.

Massie recalled that in 2013, the Obama administration claimed Assad used chemical weapons against civilians, but did not present a compelling case to Congress.

"When they gave [Congress] classified briefings ... they were not able to compel us to believe with sufficient evidence that Assad did it and we should get involved," said Massie.

Carlson asked Massie whether he was claiming Syria and Russia have "equal believability" as the United States government.

Massie said he was not making that claim, but claimed the U.S. government has "no interest in finding evidence contrary to the popular notion" that Assad carried out the attack.

Watch the full interview above.

Campus Chick-fil-A Threatens Students' 'Safe Place'? Army Ranger Sounds Off

Hannity: Media 'Sounding Like Putin' With Their Trump Conspiracy Theories

Trump on North Korea: 'We Are Sending an Armada'

'You Lie!' Chants Break Out at GOP Rep. Joe Wilson's Town Hall