Such munitions are dropped by helicopters, Higgins said, and the OPCW has determined that yellow gas cylinders have been dropped by helicopters in previous chemical weapons attacks, according to The Associated Press, which has seen the reports.

But Higgins' team was also able to determine from where the helicopters took off.

\"Aircraft observers that are part of the Sentry Syrian network observed two Hip helicopters heading southwest from Dumayr Airbase,\" Higgins wrote, mentioning a regime airbase, \"northeast of Damascus, in the direction of Douma, 30 minutes before the chemical attack in Douma, and two Hip helicopters were observed above Douma shortly before the attack.\"

But from where Higgins sits, at home in the U.K., there are still unknowns.

\"One thing that's confusing,\" he said, \"is that the doctors are all adamant that there are Sarin-like symptoms but this may be because of overexposure to chlorine.\"

But, he added, \"there does seem to be another chemical agent but I don't think it's Sarin.\"

In the analysis, he wrote \"it is important to note that these yellow gas cylinders are not associated with the use of Sarin, and as Sarin is a liquid a compressed gas cylinder seems an unlikely method of delivery for Sarin. Possible explanations for the allegations of Sarin use may be a result of the severity of the symptoms presented, of an undocumented munition being used, or another chemical agent being used that presents symptoms that could be confused with Sarin use.\"

There are limits to investigating from afar, Higgins acknowledged, which is why it's crucial that the OPCW get to the site and start testing the clothing and blood of victims. Whether they will find another kind of munition that could have carried a different chemical is an open question.

In Higgins opinion, that would be one way to explain an additional chemical and the Sarin-like symptoms.

Higgins' analysis concluded that \"based on the available evidence, it is highly likely the 34+ victims killed in the 7:30pm attack on the apartment building near al-Shuhada Square were killed as a result of a gas cylinder filled with what is most likely chlorine gas being dropped from a Hip helicopter originating from Dumayr Airbase.\"

\"The thing is with this,\" he explained, \"there have been constant chemical attacks this year. Five to six already this year. The only thing that's special about this one is that it got noticed.

\"If it was dropped just a few meters to the other direction, those people in that building probably wouldn't have died,\" he added. \"A matter of meters between a big news story or just another chemical attack.\"

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