The nerve agent sarin was used against Syrian civilians in April, according to a report from the international chemical weapons watchdog that confirms the suspicions of experts.
The Organisation for the Prohibition of Chemical Weapons says it confirmed that sarin was deployed in the rebel-held town of Khan Shaykhun, killing scores of people, including children. The organization says it reached its conclusion after attending autopsies, collecting bio-medical samples of victims, interviewing witnesses and testing samples from the environment.
It stopped short of identifying who was responsible for the attack, saying that was outside its fact-finding mandate.
The White House has said the intelligence community is confident that Syrian President Bashar Assad is responsible. The attack prompted President Trump to order strikes against a Syrian military air base.
On Monday, the White House said in a statement it has seen possible preparations for another chemical weapons attack similar to the one in April. It warned that if Assad carries out an attack, "he and his military will pay a heavy price."
"I strongly condemn this atrocity, which wholly contradicts the norms enshrined in the Chemical Weapons Convention," the watchdog organization's Director-General Ahmet Uzumcu said in a statement. "The perpetrators of this horrific attack must be held accountable for their crimes."
The U.S. State Department said the "facts reflect a despicable and highly dangerous record of chemical weapons use by the Assad regime."
It added that this report will be sent to an "independent international expert mechanism established by the UN Security Council, to determine who is responsible for the attack."
The April 4 airstrike killed scores of people, and experts immediately pointed to the possibility of sarin. Sarin is more lethal than chlorine gas, which has been documented numerous times in Syria.
Activist Samer al-Hussein saw the aftermath of the attack and described the scene to NPR's Alison Meuse:
"I saw something I'd never seen in my life. ...Dozens of children, women, men and elderly people lying on the ground, getting hosed down with water, out in the cold. Children trying to breathe a gasp of air, with saliva and foam coming out of their mouths and nostrils."
Hussein told Alison he saw "entire families being pulled from their apartments, lifeless."
The day after the attack, Doctors Without Borders said it had examined eight patients who "showed symptoms – including constricted pupils, muscle spasms and involuntary defecation – which are consistent with exposure to a neurotoxic agent such as sarin gas or similar compounds."
Turkey's health ministry also stated that its autopsies of attack victims showed evidence of sarin use.
As we have reported, U.S. officials have rejected the Russian government's claims that "a conventional weapons strike by the Assad regime accidentally hit a stockpile of chemical weapons that belonged to rebels or terrorists." Russia is allied with the Syrian regime.
NPR's Geoff Brumfiel spoke with chemical weapons expert Dan Kaszeta, who explained why Russia's claim is unlikely.
"Kaszeta says ... nerve agents are unstable and are typically stored as two separate chemicals. With sarin, for example, one of those precursor chemicals is highly flammable isopropyl alcohol.
" 'You drop a bomb on it, the whole thing is going up in a huge fireball,' he says. Even if the nerve agent was pre-mixed, a bomb strike would fail to disperse it in a way that could cause mass casualties.
"Kaszeta says he thinks the most likely source of chemical was the Syrian regime. Sarin and other nerve agents are hard to make, and it's unlikely that rebel groups would have access to it. ... The Syrian regime is still believed to have experts who could make nerve agent from scratch."
The Syrian government maintains it destroyed its chemical weapons stockpile in 2013. That was after then-President Obama said Syria had crossed a red line by attacking a Damascus suburb with sarin, killing hundreds.