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.
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.
"We know from the characteristics that this is the result of violence between criminal gangs, it is not an attack on the civilian population," Nuevo Leon state security spokesman Jorge Domene said.
Nuevo Leon's prosecutor, Adrian de la Garza, said the fact that hands and heads had been cut off made it difficult to identify the victims, but he said it was possible they were Central American migrants.
The grim find comes just days after police discovered the dismembered, decapitated bodies of 18 people in two abandoned vehicles in western Mexico.
Earlier this month 23 dead bodies - 14 of them decapitated - were found in the border city of Nuevo Laredo, also in Nuevo Leon state.
Around 50,000 people have died in drug-related violence in Mexico since 2006, when President Felipe Calderon deployed the army to combat the cartels.
The BBC's Will Grant, in Mexico City says the latest killings show that, although many Mexicans felt the drug violence had been easing this year, the conflict is still claiming many lives, often in the most brutal circumstances.
The three main candidates to succeed Mr Calderon in July's presidential election have all said they would work to end the violence, but have not offered any concrete plans.