United Nations special envoy to Syria Staffan de Mistura is seen on a video screen as he listens to a Security council meeting on Syria at U.N. headquarters in June about the possible use of sarin gas in an attack that killed more than 90 people.
Hassan Youssef, a 40-year-old victim of the April 4 chemical attack in Khan Shaykun, receives medical care in a hospital in the nearby Syrian city of Idlib in April.
Omar Haj Kadour/AFP/Getty Images
Doctors at a hospital in Aleppo, Syria, treat a boy injured in what the government said was a chemical weapons attack on March 19. Syria's government and rebels accused each other of firing a rocket loaded with chemical agents outside of Aleppo.