Rescue workers arriving at a road accident in southern Mexico found a truck jammed with as many as 200 migrants crashed into the base of a steel pedestrian bridge, killing 55 and injuring dozens.
The migrants inside the tractor trailer were tossed and crushed in a pile of both the living and the dead.
In addition to the 55 killed Thursday evening, at least 52 were injured.
It was one of the deadliest days for migrants in Mexico since the 2010 massacre of 72 people by the Zetas drug cartel in the northern state of Tamaulipas.
First-arriving rescuers said some people who had been on the truck had fled for fear of being detained by immigration agents.