New evidence supports the conclusion that Syrian government forces have used nerve agents on at least four occasions in recent months: on April 4, 2017, in a chemical attack on Khan Sheikhoun that killed at least 92 people, and on three other occasions in December 2016 and March 2017. These attacks are part of a broader pattern of Syrian government forces’ use of chemical weapons. The attacks are widespread and systematic and in some cases have been directed against the civilian population. These two features mean the attacks could meet the legal standard required to characterize them as crimes against humanity.