“The pattern of the chlorine attacks shows that they were coordinated with the overall military strategy for retaking Aleppo, not the work of a few rogue elements,” Ole Solvang, deputy emergencies director at Human Rights Watch, said in the report.

Human Rights Watch documented at least eight separate chlorine gas attacks before a cease-fire was signed Dec 13. The attacks resulted in the deaths of nine civilians, including four children, and wounded roughly 200. If confirmed, the attacks would be a significant breach of the 1993 Chemical Weapons Convention that Syria signed in 2013. Although chlorine is not considered a chemical weapon by the convention because of its industrial uses, the document explicitly states that weaponizing the chemical properties of a substance is prohibited.

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When inhaled in large enough quantities, chlorine can be fatally toxic.

The Syrian government has routinely denied using internationally condemned weapons such as chemical, cluster and incendiary munitions. In 2013, Syrian government forces were accused of using sarin gas in an attack that killed hundreds, bringing the United States to the brink of military action against Assad. In a last-minute deal coordinated by Russia, the Syrian president pledged to give up his chemical weapon stockpiles, and in October of that year signed the Chemical Weapons Convention. The Islamic State has also used chemical weapons, including variants of sulfur mustard powder.

Following the deal, however, Assad continued to use chlorine as a weapon. Last year, a United Nations investigative team determined that Syrian government forces had been responsible for three separate gas attacks in April 2014 and March 2015.

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Although the report doesn’t directly implicate Russia, it does say that Russian forces “benefited” from the strategic effects of the gas attacks. Russian forces had been providing close air support for Syrian government forces since September 2015 and flew hundreds, if not thousands, of sorties during the battle to retake Aleppo. Russian Special Operations forces also likely participated in the campaign, fighting and advising alongside Syrian troops as they pushed into the city.

The report relied on interviews with witnesses, videos, photographs and social media postings to verify the eight attacks. Some of the posts included pictures of the spent gas canisters dropped from Syrian helicopters and footage of the trademark yellow-green chlorine clouds billowing from residential areas.