UNITED NATIONS —Chemical weapons investigators concluded “with a high degree of confidence” that chlorine gas was used as a weapon against three Syrian villages last year, affecting between 350 and 500 people and killing 13, according to a report obtained Tuesday by The Associated Press.

The third report by a fact-finding mission from the Organization for the Prohibition of Chemical Weapons didn’t apportion blame but said 32 of the 37 people interviewed “saw or heard the sound of a helicopter over the village at the time of the attack with barrel bombs containing toxic chemicals.”

The investigators said 26 people heard the distinctive “whistling” sound of the falling barrel bombs containing toxic chemicals and 16 visited the impact sites and saw the bombs or their remnants. The report includes a description of 142 videos and 189 pieces of material obtained by the investigators as well as photos of impact sites and the inner chlorine cylinder from a barrel bomb.

The mission was established by the OPCW on April 29 to establish the facts surrounding allegations of the use of chlorine “for hostile purposes” in Syria. Chlorine gas is readily available and is used in industry around the world, but can also be used as a weapon.

The U.N. Security Council unanimously adopted a resolution on Sept. 27, 2013 ordering Syria’s chemical weapons stockpile to be secured and destroyed.