International investigators have said for the first time that they suspect President Bashar Assad and his brother are responsible for the use of chemical weapons in the Syrian conflict, according to a document seen by Reuters.

A joint inquiry for the United Nations and a global watchdog, the Organization for the Prohibition of Chemical Weapons, had previously identified only military units and did not name any commanders or officials.

Now a list has been produced of individuals whom the investigators have linked to a series of chlorine bomb attacks in 2014 and 2015 — including Assad, his younger brother Maher and other high-ranking figures — indicating the decision to use toxic weapons came from the very top, according to a source familiar with the inquiry.