The authorities in Mexico have found 13 more bodies, bringing to 72 the number found in mass graves uncovered in northeastern Mexico in recent days.
Eight mass graves were found near the town of San Fernando on Wednesday, and two more were found by Friday, Tamaulipas state Interior Secretary Morelos Canseco said Friday.
Investigators have been chasing reports that gunmen had kidnapped people off buses headed toward the U.S. border.
Nine of the newly discovered bodies were found in one grave and four in another on Thursday near the city of San Fernando. All were male.
San Fernando, the same rural town — 90 miles from the Texas border — where 72 South American migrants were killed last August. They were allegedly killed by the gang Los Zetas.
The Mexican authorities reportedly said the Zetas gang, one of several major drug trafficking outfits operating in Mexico, carried out the killings after the migrants refused to work for them.
Fourteen suspects have been arrested so far in connection with the mass graves, in raids that also led to the rescue of five kidnapping victims, DPA reports.
Police began investigating after a complaint was filed on March 25 over the alleged kidnapping of a number of people traveling in a public bus. Forensic tests are being conducted.
Thousands of angry citizens took to the streets of 38 Mexican cities on Wednesday to protest escalating drug violence that has killed more than 34,600 people since December 2006.
"These reprehensible acts underline the cowardice and the total lack of scruples of the criminal organizations, which generate violence in our country, and especially in the state of Tamaulipas," the office of Mexican President Felipe Calderon said in a statement, CNN reported.