Mexican authorities announced Friday the discovery of 13 more bodies in the violence-torn border state of Tamaulipas, where 59 bodies were unearthed in eight pits earlier this week.
It was not immediately clear if the latest two graves, found Thursday, were related to the others.
The 13 bodies, all men and thought to be Mexican, were discovered in a different spot than the other graves, a state official said. Authorities found the previous bodies while investigating mass kidnappings of passengers from buses passing through the area.
Last year, 72 migrants from Central and South America were found slain on a remote ranch in the same region. That massacre was blamed on the Zetas, an ultra-violent drug gang that engages in migrant-smuggling, extortion and kidnapping.
Tamaulipas officials have only begun identifying the latest bodies. Preliminary evidence suggests that the 59 bodies found earlier were of Mexicans, not foreign migrants, officials said.
The discoveries have added to a sense that Tamaulipas, for decades a drug-smuggling corridor and now scene of a bloody feud between the Zetas and former allies, has slipped from the control of Mexican authorities.
The area is along a highway that serves as a path for U.S.-bound migrants from Mexico and Central America.
Morelos Jaime Canseco, government secretary for Tamaulipas, said officials have been in touch with authorities in other parts of Mexico, including the central states of Guanajuato and Queretaro, where residents have reported the disappearances of loved ones who went missing on their way north.
Border-bound buses from the north-central city of San Luis Potosi could have been carrying people from all over Mexico, he said.
"We don't know if their destination was to try to get into the United States to work in that country; that is a circumstance that will have to be cleared up," Canseco said in a television interview.
Thousands of Mexicans have disappeared since President Felipe Calderon declared war on drug traffickers four years ago and violence began to spiral nationwide.
On Thursday, government spokesman Alejandro Poire blamed a "criminal cell" for the 59 deaths, but did not say which one. He also announced that authorities had arrested 14 people in connection with a March 25 bus kidnapping and freed five victims.