WASHINGTON >> The United States, Britain and France opted to strike Syria for its apparent use of chemical weapons without waiting for a report from U.N. inspectors because they were convinced that the Assad government had used chlorine and sarin nerve gas against a rebel-held Damascus suburb, American officials said today.

The allies also acted because of concerns that Russian and Syrian forces may already have tried to clean up important evidence in Douma, where more than 40 people died in last weekend’s attack, the officials said.

The three countries launched their missiles even as the fact-finding team from the Organization for the Prohibition of Chemical Weapons was in the Syrian capital and had been expected to head today to Douma.

Russia and Syria have denied that chemical weapons were used at all and said their own investigators had been to the area and found no trace of them. Those assertions have been denounced as lies by Western officials.

The West’s assessments of what happened April 7 in Douma rely mainly on open source information. That includes witness testimony, as well as video and photos shot by aid workers, victims of the attacks and unspecified additional intelligence about barrel bombs and chlorine canisters found in the aftermath.

Barrel bombs are large containers packed with fuel, explosives and scraps of metal, and British Prime Minister Theresa May said reports indicated the Syrian government had used one to deliver the chemicals.

The White House said doctors and aid organizations on the ground in Douma reported “the strong smell of chlorine and described symptoms consistent with exposure to sarin.” A senior administration official told reporters Saturday that while there was more publicly available evidence pointing to the use of chlorine, the U.S. has “significant information that also points to sarin use.”

The official did not elaborate on what that information entailed.

Chlorine use has been a recurring footnote in the course of Syria’s civil war, but rarely has it generated the same outrage as reports of sarin use.

Chlorine has legitimate industrial and other civilian uses, so it is not banned by the Chemical Weapons Convention. The treaty does, however, prohibit the use of chlorine as a weapon.

One senior U.S. official familiar with the decision to act on Friday said the U.S., British and French intelligence services were unanimous in their assessments of the attack and were “eager” to move when they did because of concerns about contamination of the site.

The officials spoke on condition of anonymity because they were not authorized to publicly discuss specifics beyond those contained in the formal statements.

Despite the strikes, the chemical weapons watchdog agency said its experts would go ahead with their mission. The Russian foreign ministry, however, accused the allies of acting when they did “to hamper the work of the OPCW inspectors.”

The U.S. has denied that assertion and called the group’s mission “essential” to a complete understanding of what chemical agents were used.

A second U.S. official said Britain, France and the U.S. are confident that the inspectors’ eventual report will confirm their findings that chlorine was used, likely in conjunction with sarin.

The three governments noted dozens previous, smaller-scale chlorine and other chemical weapons attacks over the course of the past year, since President Donald Trump first ordered airstrikes against Syria last April.

Reports of major chlorine attacks began emerging in 2014, soon after Syria’s declaration of complete chemical disarmament, which was the result of an Obama administration agreement between the U.S. and Russia. The agreement only covered declared chemical weapons. Syria is widely suspected of hiding some stocks, manufacturing more as well as holding on to chlorine.

“The pictures of dead children were not fake news. They were the result of the Syrian regime’s barbaric inhumanity,” Trump’s U.N. ambassador, Nikki Haley, said Saturday. “And they were the result of the regime and Russia’s failure to live up to their international commitments to remove all chemical weapons from Syria. The United States, France, and the United Kingdom acted after careful evaluation of these facts. “

In August 2015, the U.N. Security Council first authorized the OPCW and U.N. investigators to probe reports of chemical weapons use in Syria, as witnesses began to circulate increasing accounts of chlorine attacks by government forces against civilians in opposition-held areas.

A year later, the joint OPCW-U.N. panel determined the Syrian government had twice used helicopters to deploy chlorine against its opponents in civilian areas in northern Idlib province. A later report held the government responsible for a third attack.

There have been dozens of attacks with chlorine gas since then, including an attack in Aleppo in 2016 that reportedly killed a woman and two children, and at least two attacks on the town of Saraqeb in northern Syria that injured dozens.