Our assessment is it is, at a minimum, concentrated chlorine dropped from helicopters, that could only be the regime.The following are playlists of videos from various attacks that have been reported since April 11th
April 11th - Kafr Zita, Hama
April 12th - Al-Taman'ah, Idlib
April 14th - Atshan, Hama
April 16th - Kafr Zita, Hama
April 18th - Al-Taman'ah, Idlib
April 18th - Kafr Zita, Hama
April 21st - Telmans, Idlib
It should be noted these playlists represent only some of the reported chlorine attacks over the past 10 days, with at least six having been reported in Kafr Zita over that period. Chemical attacks were also reported in Harasta, Damascus on April 11th and April 16th. It should be stressed there's been no way to independently verify these reports.
Along with videos of victims, videos showing the remains of the improvised barrel bombs used in the attack have also been posted online (playlist). These videos share some similarities with each other, which adds some weight behind the claim chlorine gas was used. The following video from Kafr Zita shows the remains of two barrel bombs reportedly used in the attack, with the remains of what appear to be chlorine cylinders are part of the debris
It's worth noting the yellow paint on the chlorine cylinders, the colour coding used for industrial chlorine cylinders in many parts of the world.
Two days after this video was posted online, another video from Kafr Zita was uploaded showing the remains of what was claimed to be another chlorine barrel bomb
In this case it appears explosive det cord has been wrapped around the neck of the gas cylinder, no doubt in an attempt to explosively detach it from the cylinder on impact. The colours on this cylinder are interesting as well, it appears to be a yellow undercoat with red painted on top. It's not entirely visible, but it could possibly be an ammonia cylinder, which is usually painting yellow with a red top.
Videos from the April 21st attack at Telmens (playlist) also show the remains of barrel bombs, as well as dead animals near the impact sites
It appears in the first video we can yet again see yellow paint on a cylinder inside the barrel bomb. Neither video is terribly clear, but there do appear to be some small clues chlorine was used again.