Mexican authorities have pulled dozens of corpses from mass graves in a state near the U.S. border, with the latest body count standing at 145.
Authorities have arrested 16 municipal police officers accused of protecting drug gang hitmen believed to be behind the Tamaulipas state massacres.
Mexican officials say the Zetas drug gang is responsible for the killings. They say they have arrested 17 members of the gang so far.
At least some of the victims appear to have been passengers on buses traveling through the area.
Migrants trying to reach the United States often cross through Tamaulipas. The state is where 72 Central American migrants were found shot to death last August. The Zetas have been blamed for that massacre as well.
Drug-related killings have claimed at least 35,000 lives since 2006, when President Felipe Calderon took office and launched an offensive against the cartels.
The Zetas began as a Mexican military unit that defected and began working with the Gulf cartel, based in Juarez, Mexico, across the Rio Grande River from the U.S. city of El Paso, Texas. The Zetas split from the Gulf cartel last year. The two groups are now fierce rivals.
Some information for this report was provided by AP, AFP and Reuters.