It was rebels who stored chemical weapons in the Khan Sheikhoun warehouse. Syrian and Russian forces attacked it and therefore people were hit by the two chemical weapons: sarin and chlorine.
U.N. Syria mediator Staffan de Mistura said the "horrific" chemical attack had come from the air. http://reut.rs/2nEvlZW The Director-General of the Organisation for the Prohibition of Chemical Weapons (OPCW) has informed that the OPCW-United Nations Joint Investigative Mechanism was gathering and analysing information from all available sources, and would be prepared to deploy a team at the earliest opportunity. He said the attack had reportedly been carried out through an air strike on a residential area, but the means of delivery could not be confirmed. http://bit.ly/2nc6vUY. Russia has vetoed the most recent UN Security Council resolution that would force Assad to cooperate with an international investigation; it was already the eighth time that Russia has vetoed a resolution regarding Syrian war http://cnn.it/2puhrtD.