Forty-nine mutilated bodies have been found dumped by a roadside near the city of Monterrey in northern Mexico.
Security officials said the 43 men and six women had been decapitated and had their hands cut off, making identification difficult.
They blamed the killings on a conflict between rival drugs gangs - a note left with the bodies said they had been killed by the Zetas cartel.
It is the latest in a series of recent massacres in northern Mexico.
The Zetas have been fighting the Gulf and Sinaloa cartels for control of smuggling routes into the US.Unidentified
The bodies were found at 04:00 local time (09:00 GMT) in Cadereyta municipality on the road from Monterrey to Reynosa on the US border.
Security officials said the bodies. some of which were in plastic bags, appeared to have been killed at another location up to two days ago and dumped from a truck.