(CNN) -- Eight mass graves were uncovered in northeastern Mexico Wednesday, holding at least 59 bodies, the Tamaulipas state attorney general's office said.
Authorities found the graves during an investigation into a report of the kidnapping of passengers from a bus in late March. The investigation led them to the town of San Fernando, the same place where in August of last year the bodies of 72 immigrants were found at a ranch.
This time, authorities arrested 11 suspects and rescued five hostages, the state attorney general's office said.
They also discovered the eight mass graves. There were 11 bodies found in the first six graves, 43 bodies in the seventh, and five in the eighth, the agency said.
Forensic investigators will examine the bodies in an attempt to identify them and to see if they are the missing bus passengers.
Tamaulipas Gov. Egidio Torre Cantu condemned the violence, and said he would collaborate with federal authorities to locate and punish those responsible.
"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.
Amnesty International also called on Mexico to fully investigate, and it criticized the country's efforts to protect its citizens and migrants passing through.
"The mass graves found yesterday once again 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, abductions and killings," said Rupert Knox, a researcher on Mexico at Amnesty International. "All too often such human rights crimes have gone unpunished, leaving criminal gangs and officials acting in collusion with them free to target vulnerable communities, such as irregular migrants."
The nationalities of the victims found in the mass graves were not immediately known.
Tamaulipas is one of Mexico's most active states when it comes to drug trafficking activity. The Gulf cartel and the Zetas cartel both operate in the state and have strongholds there.
The Zetas have been blamed for the killings of the 72 migrants who were found in San Fernando last year.