A unit of elite Syrian troops was responsible for a 2018 chemical weapons attack that killed 43 civilians in the Syrian town of Douma, the global chemical weapons watchdog said Friday.

The April 7 attack was part of a brutal military offensive by Syrian President Bashar al-Assad's troops as they forced rebel fighters from the outskirts of Damascus. Photographs that showed men, women and children dead in a stairway began circulating online in the early hours of the next morning. Video footage showed others choking or foaming at the mouth.

The United States, Britain and France retaliated days later, launching a rare salvo of airstrikes against Syrian government targets. But in the months that followed, Syria and its Russian allies vehemently denied that Assad's forces had been responsible for any chemical attack. International weapons inspectors struggled to access the two sites in question and the incident became a magnet for Russian-backed disinformation.

A previous effort by the Organization for the Prohibition of Chemical Weapons (OPCW) had already concluded that a chemical attack had taken place in Douma, but had no mandate to assign blame.

In the organization's 139-page report Friday, it described an exhaustive effort to do just that — investigators combed through 1.86 terabytes of data, took 66 witness statements and examined data from 70 samples.

"On the evening of 7 April 2018, at least one helicopter of the Syrian 'Tiger Forces' Elite Unit dropped two yellow cylinders containing toxic chlorine gas on two apartment buildings in a civilian-inhabited area in Douma, killing 43 named individuals and affecting dozens more," the OPCW concluded.

The United Nations says over 300,000 people have been killed during Syria's more than decade-long civil war. The true number is probably much higher. Despite extensive evidence implicating Assad's top brass in likely war crimes, human rights lawyers say international tribunals are unlikely to ever try them because of a lack of jurisdiction.

Researchers say Assad's military and affiliated militias have launched more than 300 chemical attacks throughout the war, with the weapons ranging from nerve agents to crude but dangerous chlorine bombs.

"The Assad regime did not merely 'get away' with its use of these banned weapons," the Berlin-based Global Public Policy Institute said in 2019. "It succeeded in using them for strategic ends."

As in the aftermath of other chemical strikes, the April 2018 attack paved the way for Assad's army to force a punishing surrender by rebels.

"The use of chemical weapons in Douma — and anywhere — is unacceptable," said OPCW Director General Fernando Arias. "The world now knows the facts — it is up to the international community to take action."

The Investigation and Identification Team, which wrote the report, was established by member states at the Hague-based OPCW in November 2018, after Russia vetoed a joint U.N.-OPCW mission to Syria.

Although investigators modeled tens of thousands of possible trajectories for two yellow cylinders that appeared in photographs shortly after the strike, they were unable to examine the weapons themselves. The Syrian government, it said, notified the OPCW in 2019 that it had destroyed the containers.

"The destruction of the cylinders has precluded any further assessment or analysis for the purposes of the present report," the OPCW said.

The watchdog said it had received credible information indicating that the helicopter squad that departed from Dumayr air base between 7:10 and 7:40 p.m. on April 7, 2018, was likely under the control of one of Syria's top military officers, then-Brig. Gen. Souheil al-Hasan.

The attack swiftly became part of a disinformation campaign by the Russian state and a number of high-profile online activists. In lengthy blog posts and in podcasts, they claimed that the evidence was fabricated, that corpses were arranged at the site, and that children seen foaming at the mouth were faking their symptoms.

On Friday, the OPCW's report dismissed those theories one by one, providing extensive refutations in each case.

"At both locations, it would have been extremely difficult, if not impossible, to mimic the spread of a gas such as chlorine gas," the report concluded, adding that investigators were "unable to identify any evidence, including from opensource information or from the Syrian Arab Republic or other States Parties, which would corroborate that any of the aforementioned staging actions were performed at either location."

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