US President Donald Trump lauded the "perfectly executed" strike, the biggest worldwide attack on President Bashar al-Assad's regime during Syria's seven-year war, but both Damascus and Syria's opposition rubbished its impact.
The OPCW itself had declared that the Syrian government's chemical weapons stockpile had been removed in 2014, only to confirm later that sarin was used in a 2017 attack in the northern town of Khan Sheikhun.
The inspectors will face a hard task, with all key players having pre-empted their findings, including Western powers, which justified the strikes by claiming they already had proof such weapons were used.
The three countries consulted and collaborated on an assault in which more than a 100 missiles, reportedly targeting specific sites, were launched into Syria after the government was accused of a chemical attack.
"On the intelligence collected by our services, and in the absence to date of chemical samples analysed by our own laboratories, France considers, beyond possible doubt, a chemical attack was carried out against civilians at Douma ... and that there is no plausible scenario other than that of an attack by Syrian armed forces", the report said.
Trump Suggests He Is Making The Phrase 'Mission Accomplished' Great Again
When pressed, he acknowledged that some unspecified portion of Assad's chemical arms infrastructure was not targeted. Bush for the last six years of his presidency, and became a shorthand for a premature declaration of victory.
The first target was a scientific research center located in the greater Damascus area, which Mattis said was used for "the research, development, production, and testing of chemical and biological warfare technology". "We sent a clear message to Assad and his murderous lieutenants that they should not perpetrate another chemical weapons' attack for which they will be held accountable".
Worldwide officials are working to try to confirm the suspected poison gas attack on Douma.
Inspectors from the Organisation for the Prohibition of Chemical Weapons have met Syrian Deputy Foreign Minister Faisal Mekdad.
The US military said the strikes "destroyed" Him Shinshar's chemical weapons storage facility, and damaged the chemical weapons bunker.
Russian Federation 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.
A United Nations-OPCW Joint Investigative Mechanism also looked at responsibility for chemical weapons use. It blamed Islamic State militants for mustard gas use. Damascus and Moscow deny blame for any such attack.
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