Human Rights Watch (HRW) says new evidence indicates that the Syrian government used nerve agents in four chemical-weapons attacks since December, including the one in Khan Sheikhun on April 4 that killed nearly 100 people, including 30 children.
The rights group said in a new report issued on May 1 that these attacks "are part of a broader pattern of Syrian government forces' use of chemical weapons" that may constitute crimes against humanity.
"The government's use of nerve agents is a deadly escalation -- and a part of a clear pattern," Kenneth Roth, HRW's executive director, said.
The rights group said it was not able to conduct ground investigation of the attack sites.
But it interviewed 60 witnesses and reviewed photos and videos of impact sites and victims that were posted online and provided directly by local residents.