Syrian authorities have killed at least 13,000 people since the start of the 2011 uprising in mass hangings at a prison north of Damascus known to detainees as “the slaughterhouse,” Amnesty International said in a report Tuesday. It covers the period from 2011 to 2015, when Amnesty said 20-50 people were hanged each week at Saydnaya Prison in killings authorized by senior Syrian officials, including deputies of President Bashar Assad, and carried out by military police.