Investigators found four new graves in San Fernando, not far from the United States border.
They were tipped-off by a suspect who was detained on Saturday.
Police said he had confessed to the kidnapping and subsequent killing of dozens of victims, who were travelling through the area on buses.
On Thursday, police found 59 bodies in eight mass graves in San Fernando, in Tamaulipas state. Thirteen more bodies were discovered the following day in two other graves.
Officials say 16 people have been arrested in connection with the discovery of the mass graves, but the motive for the killings remains unclear.
The gruesome find resembles the discovery last August of the bodies of 72 Central and South American migrants, who were killed in the same town for refusing to join the ranks of the cartel which had abducted them.
Tamaulipas state, where the mass graves where found, is at the centre of a bloody battle between rival drug gangs for control of the lucrative drug-smuggling routes to the US.
Around 35,000 people have been killed in drug-related violence in Mexico since President Felipe Calderon declared war on the country's drug cartels.
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