#1. Aerosolization theory: The expert said the way the agent was dispersed, according to witnesses in the northwestern town of Khan Sheikhun, was consistent with an phenomenon known as aerosolization.
Such an effect can only be caused by a military-grade chemical weapon and cannot be provoked by an accidental explosion, Lepick said.
"When you look at the areas of impact and the way people have been affected, you can see clearly that this is a munition that has been designed to deliver a chemical agent, in other words to produce an explosion, with a high degree of aerosolization," he said.
"In other words, it was the most effective way of dispersing the chemical agent."
If an airstrike had hit a warehouse where the toxic agent was being stored, it would have created a "toxic cloud", but the agent would not have been effectively dispersed into the air, he said.
#2. Explosion theory: However, Julien Legros, a researcher at France's national scientific research centre (CNRS) said he believed the Russian theory was not "out of the question".
He argued that it was theoretically possible that the contents of the toxic agent were stored in the same barrel and an explosion caused by an air strike could have caused a chemical reaction which turned the gas into a toxic gas.
A large part of the substance would have b een destroyed in such a strike, "but if even 10 percent of it had become vapour, that 10 percent would have been deadly".
"If 10 milligrammes is enough to kill a person and you have 10 percent of a tonne, then you have more than enough to kill several hundred people," he said.