Ahmet Uzumcu, director general of the Organization for the Prohibition of Chemical Weapons (OPCW), highlighted that attacks involving chlorine barrel bombs and deadly nerve agent sarin have continued despite the landmark agreement that won the group a Nobel Peace Prize in 2013.

Syrian “authorities have to explain in a plausible, technically plausible manner why the samples collected by our experts did prove the existence of certain chemicals which were never declared,” Uzumcu said in an interview with NBC News. “So they should explain why those chemicals were present in such places.”

According to Uzumcu, the use of sarin in February 2017 raised concerns that Syria might have kept certain banned substances despite Assad’s government saying it surrendered its stockpile of chemical agents in 2013. Syria could have also redeveloped or reacquired them.