CIUDAD JUAREZ, Mexico (AP) — After migrants in northern Mexico placed mattresses against the bars of their detention cell and set them on fire, guards quickly walked away and made no apparent attempt to release the men before smoke filled the room and killed 38 men, surveillance video showed Tuesday. The high level of frustration in Ciudad Juarez was evident earlier this month when hundreds of mostly Venezuelan migrants tried to force their way across one of the international bridges to El Paso, acting on false rumors that the United States would allow them to enter the country. At a Mass celebrated in memory of the migrants, Bishop Mons