President Donald Trump ordered the bombing of a Syrian military base Thursday, less than three days after civilians died nearby from apparent exposure to chemical gas. But some congressional lawmakers have expressed concern about the factual basis for the attack.

American officials blamed Syrian President Bashar Assad for the release of chemical weapons, but Syrian officials said the poison gas actually was disseminated after military planes dropped conventional bombs on the arms depot of an al-Qaida offshoot.

Lawmakers who question the evidentiary support for Trump's missile strikes said it's plausible but unproven that Assad is responsible.

"I think we all can assume Assad was responsible, but that's an assumption at this time," Rep. Walter Jones, R-N.C., told U.S. News a couple hours before the strikes.

"I think the rush to judgment ought to be slowed," he said. "I think it probably is the Assad government, but we don't know as an absolute."

Rep. Barbara Lee, D-Calif., told U.S. News she, too, wanted more evidence, and also a greater role for Congress in debating military policy and for the United Nations in bringing an end to the long-running civil war.

"All signs are pointing now to the Assad government perpetrating these terrible, heinous attacks. But I don't know. We have to insist on an investigation, but we have to move quickly, because there are conflicting reports on this," Lee said shortly before the strikes.

Lee suggested the U.N. should do a quick investigation.

There's context for wanting proof: The 2003 invasion of Iraq was justified by bad intelligence about weapons of mass destruction, and the leader of a U.N. probe into a chemical attack near Damascus in 2013 told U.S. News he was unsure of who was responsible for the assault. The U.S. also blamed that attack on Assad, nearly leading to military action.

Lawmakers who acknowledge a sliver of doubt about whether Assad was responsible appear to be in a clear minority. Most treat Assad's guilt as a fact, and skeptics are among the loudest advocates for congressional input in warmaking decisions.

Rep. Tulsi Gabbard, a Hawaii Democrat who recently met with Assad, said in a statement after the strikes that Trump "acted recklessly" and "without waiting for the collection of evidence from the scene of the chemical poisoning."

"If President Assad is indeed guilty of this horrible chemical attack on innocent civilians, I will be the first to call for his prosecution and execution by the International Criminal Court," Gabbard said. "However, because of our attack on Syria, this investigation may now not even be possible."

Some members of Congress who demand legislative approval for military action, however, believe there's no room for doubt about Assad's culpability.

Rep. Jim Himes, a Connecticut Democrat who says he's angry Trump did not seek congressional approval first, says, "I think the Russian statements and the Assad statements about their not doing this are just utter lies."

"As a member of the [House] intelligence committee I have a pretty good feel for how relatively straightforward attribution is in cases like this," Himes says. "I have not actually been presented the evidence, but I know how they do it."

Nonetheless, Himes says, "It makes my skin crawl a little bit to think we might be in danger of finding ourselves in another Middle Eastern military conflict."

Meanwhile, Rep. Peter Welch, D-Vt., says he's more concerned about Trump's plan for future military actions than in the facts preceding Thursday's strikes.

"Oftentimes what's reported as the basis of an action, when all intelligence is in, turns out to be incorrect," he says. "[But] you don't need a lot of intelligence to know the Assad regime is evil and has used violence and terror to intimidate its own citizens, so in that respect there's a lot of evidence pre-existing that Assad is capable of doing something like this."