Authorities announced Friday that they had found 13 more bodies buried in mass graves in northern Mexico where earlier this week they exhumed 59 corpses hidden in shallow pits.

All of the bodies are being sent to the forensic morgue in the border city of Matamoros for autopsies, as investigators struggle to learn who the victims are, where they were abducted and who killed them.

Gunmen have been stopping buses traveling north to the U.S. border, then walking the aisles, pointing out male passengers and taking them away, according to surviving passengers and transit companies.

Soldiers rescued five men Sunday who had been abducted, and the military captured 14 suspected kidnappers. The arrests led authorities to the mass graves, federal security council spokesman Alejandro Poire said Thursday, condemning “these reprehensible acts.”

Morelos Canseco Gomez, a top security official in the northern state of Tamaulipas, told reporters that authorities did not know the motives for the kidnappings and murders.

Criminal gangs have sometimes abducted migrants heading toward the United States and forced them to carry drugs. The men also could have been kidnapped for ransom.

The bodies of 72 migrants who had been executed were found in August at a ranch near the site where the mass graves were discovered this week, on the outskirts of the city of San Fernando, Tamaulipas, about 90 miles south of Brownsville, Tex. Those victims included men and women from El Salvador, Honduras, Guatemala, Ecuador and Brazil.

Mexican officials say the evidence suggests the men buried in the shallow graves were Mexican.

“The investigation is in progress and we must be very careful, especially not to cause alarm among the population that is worried about the whereabouts of their loved ones, but we can deduce that, unfortunately, that they are fellow Mexicans,” Canseco said.

Amnesty International urged officials to conduct a full investigation and stated that the mass graves “show the Mexican government's failure to deal with the country's public security crisis and reduce criminal violence which has left many populations vulnerable to attacks.”

Protests took place in cities across Mexico this week, with marchers condemning the crime gangs and the inability of politicians to stem the violence.

Speaking Friday at an event for the Mexican navy in Acapulco, President Felipe Calderon said soldiers and marines would continue to play a leading role in the fight against the cartels until there were well-trained, well- equipped, honest police forces throughout the country.

The U.S. State Department on Friday issued its annual human rights report on Mexico. The document, based on cases presented by Mexico’s National Human Rights Commission, criticized Mexico’s police and military for “unlawful killings by security forces; kidnappings; physical abuse; poor and overcrowded prison conditions; arbitrary arrests and detention; corruption, inefficiency, and lack of transparency that engendered impunity within the judicial system; confessions coerced through torture.”

Nick Steinberg of the group Human Rights Watch called the report “damning” and said the U.S. Congress should press Mexico to improve. He also said the disappearances and mass graves were a symptom of weak institutions.

“There is often zero investigation,” Steinberg said. “When a family shows up to report their son has gone missing, the first question from the state is always, ‘What was your son involved in?’ ”