Forty-nine mutilated bodies have been found wrapped in plastic bags near the northern Mexican city of Monterrey.
The bodies of 43 men and six women were found dumped on a road 110 miles (180 kilometres) from the US border.
The killings appear to be the latest in a string of brutal murders linked to feuding drug cartels.
Tens of thousands of people have died in Mexico since 2006, when President Felipe Calderon deployed the army to combat these gangs.
The discovery, near the town of San Juan at 04:00 local time (09:00 GMT), led the authorities to close a highway from Monterrey to the border city of Reynosa.
The grim find comes just days after police discovered the dismembered, decapitated bodies of 18 people in two abandoned vehicles in western Mexico.
In September 2011, 35 corpses were found in the city of Veracruz, while 26 were discovered in Guadalajara in November.
The BBC’s Will Grant, in Mexico City says there is not yet any indication as to which drug gang might have carried out the latest attack.
He adds that the latest killings suggest that, despite the fact many Mexicans felt the drug violence had been easing this year, the conflict is still claiming many lives, often in the most brutal circumstances.