BEIRUT — Scores of muddied and waterlogged gunshot victims, most of them men in their 20s and 30s, were found dead in a suburb of Syria’s contested northern city of Aleppo on Tuesday. Insurgents and the government accused each other of carrying out the killings in what appeared to be the latest civil war atrocity.

Video posted by opponents of President Bashar Assad seemed to show that many had been shot in the back of the head while their hands were bound.