MEXICO CITY (AP) -- Fifty-nine bodies were found buried Wednesday in a series of pits in the northern Mexico state of Tamaulipas, near the site where suspected drug gang members massacred 72 migrants last summer, officials said.
Security forces investigating reports that a passenger bus had been hijacked in the area conducted a raid that netted 11 suspected kidnappers and freed five kidnap victims.
Then they made a grisly discovery - a total of eight pits, containing a total of 59 corpses. One of the pits held 43 dead.
The bodies are being examined to determine whether they were bus passengers who were reportedly abducted March 25, the Tamaulipas state government said in statement in which it "energetically condemned" the crimes.
The statement did not identify what drug gang, if any, that the 11 arrested suspects belonged to, or why they might have hijacked the bus.
President Felipe Calderon's office issued a statement saying the find "underlines the cowardliness and total lack of scruples of the criminal organizations that cause violence in our country."
While there was no immediate confirmation that a drug cartel was involved, officials refer to the cartels as "criminal organizations."
The statement said Calderon had ordered federal officials to help in the investigation, and particularly in the work of identifying the victims.
The pits were found in the farm hamlet of La Joya in the township of San Fernando, in the same area where the bodies of 72 migrants, most from Central America, were found shot to death Aug. 24 at a ranch.
The area is about 80 miles (130 kilometers) from the border at Brownsville, Texas.
Authorities blamed that massacre on the Zetas drug gang, which is fighting its one-time allies in the Gulf cartel for control of the region.
The victims in the August massacre were illegal immigrants from El Salvador, Honduras, Guatemala, Ecuador and Brazil. An Ecuadorean and Honduran survived the attack, which Mexican authorities say occurred after the migrants refused to work for the cartel.
Mexican drug cartels have taken to recruiting migrants, common criminals and youths, Mexican authorities say.
It was unclear if the victims found Wednesday were migrants. Migrants frequently travel by bus in Mexico/
But drug gunmen also operate kidnapping rings, and erect roadblocks on highways in Tamaulipas and other northern states, where they hijack vehicles and rob and sometimes kill passengers.
San Fernando is on a major highway that leads to the U.S. border.
Drug gangs across Mexico also sometimes use mass graves to dispose of the bodies of executed rivals.
The wave of drug-related killings - which has claimed more than 34,000 lives in the four years since the government launched an offensive against drug cartels - drew thousands of protesters into the streets of Mexico's capital and several other cities Wednesday in marches against violence.
Many of the protesters said the government offensive has stirred up the violence.
"We need to end this war, because it is a senseless war that the government started," said protester Alma Lilia Roura, 60, an art historian.
Several thousand people joined the demonstration in downtown Mexico City, chanting "No More Blood!" and "Not One More!" A similar number marched through the southern city of Cuernavaca.
Parents marched with toddlers, and protesters held up signs highlighting the disproportionate toll among the nation's youth. "Today a student, tomorrow a corpse," read one sign carried by demonstrators.
The marches were spurred in part by the March 28 killing of Juan Francisco Sicilia, the son of Mexican poet Javier Sicilia, and six other people in Cuernavaca.
"We are putting pressure on the government, because this can't go on," said the elder Sicilia. "It seems that we are like animals that can be murdered with impunity."