Samples from a deadly sarin attack in Syria bear “the signature” of President Bashar al-Assad’s chemical weapons program and point to its use in other massacres, French officials said Wednesday. 

The announcement marks the strongest evidence yet that Assad’s government was responsible for the daybreak attack on the northwestern Syrian town of Khan Sheikhoun earlier this month. At least 86 people were killed, many of them as they slept. 

French Foreign Minister Jean-Marc Ayrault announced the results of an independent French investigation into the April 4 attack on the town in Idlib province.

“There is no doubt about the use of sarin,” Ayrault said. “The responsibility of the Syrian regime can no longer be doubted.” Hours later, the French government released a dossier compiled by intelligence officials stating that sarin samples from the attack site showed that the nerve agent was produced “according to the same manufacturing process” used in an earlier attack attributed to Assad’s armed forces. 

The discovery of sarin, a banned chemical agent, has been corroborated through independent tests by the Organization for the Prohibition of Chemical Weapons, as well as British and Turkish forensic scientists.

In this April 4 photo, Abdel Hameed Alyousef, 29, holds his twin babies, who were killed during a chemical attack in Khan Sheikhoun, Syria. (AP)

The nerve agent forces the nervous system into overdrive and can kill within minutes.

Although the Syrian government was supposed to have surrendered stocks to international inspectors in 2013, investigators and Western diplomats have long suspected that stockpiles were secretly withheld or that new batches were produced. 

The French dossier, published online Wednesday, appeared to offer strong indications of the Assad government’s long-denied involvement in other deadly attacks. 

Images of children convulsing and foaming at the mouth after the Khan Sheikhoun attack stirred President Trump to launch the first American military action against Assad’s government, with the United States firing dozens of cruise missiles at an air base in central Syria from which the warplanes had departed.

There was no immediate response from Syria on the French report. But Assad has denied that his government has ever used chemical weapons against its own people. In the hours after the April 4 attack, it claimed the sarin emanated from a rebel-run chemical factory after it was bombed by the Syrian air force. 

There is little known evidence to support that claim. On the morning of the attack, a network of civilian observers issued an alert as Syrian warplanes took off from the nearby Shayrat airfield and headed for Khan Sheikhoun. 

As the aircraft circled in the sky, an observer radioed colleagues to warn of an imminent attack. “Guys, tell people to wear masks,” the observer said, according to a transcript. “It has chemicals with it. I am sure of that.”

Chemical-weapons experts say the Syrian government has used its supplies of toxic agents primarily to depopulate civilian areas and strike fear into those who remain there.

The pockets of residents left behind in Khan Sheikhoun have been subjected to repeated bombing raids since the April 4 attack. On Monday, the Syrian Observatory for Human Rights monitoring group said a missile killed seven people in the town’s newly reopened market, scattering bodies and limbs among cartloads of spoiled fruit. 

Experts and monitors said the French report also appeared to offer near-definitive proof of the Syrian government’s involvement in an August 2013 sarin attack on the Damascus suburbs. Those strikes killed more than 1,000 people and pushed the Obama administration to the brink of military action against Assad.

The report identified the use of hexamine in Khan Sheikhoun, a chemical that it said was manufactured in the same way as samples found in an earlier attack in April 2013. 

In the August 2013 attacks — the deadliest of Syria’s war — chemical-weapons inspectors also found hexamine. 

“With France making a connection between those two attacks because of the presence of hexamine, it would stand to reason the same connection exists with the August 21st, 2013, sarin attacks,” said Eliot Higgins, a Britain-based researcher who founded the investigative website  Bellingcat.

Speaking to reporters Wednesday, Kremlin spokesman Dmitry Peskov said that Russia’s position on the attack is “unchanged” and that “the only way to establish the truth about what happened near Idlib is an impartial international investigation.”

In an interview with The Washington Post this week, a former head of Syria’s chemical-weapons program said the orders to use sarin could only have come from the highest level.

“The chain of responsibility is always clear,” said Brig. Gen. Zaher al-Sakat, who served in the army’s 5th Division until his defection in 2013.

“The order to use a nerve agent has to come from the Presidential Palace, and in this case that’s Assad,” Sakat said. “If it’s a lower-level chemical like chlorine, then field commanders can give the order. But for something like this, it’s Assad.”

McAuley reported from Paris.