At Hospital 601, not far from the presidential palace in Damascus, Syrian guards ran out of space to store the dead and had to use an adjoining warehouse where military vehicles were repaired.

A forensic photographer working for Syria's military police walked the rows and took pictures of the emaciated and disfigured corpses, most believed to be anti-Assad activists. Numbers written on the bodies and on white cards, the photographer said, told regime bureaucrats the identities of the deceased, when they died and which branch...