MEXICO CITY – More than 40 bodies have been found in a mass grave in the northern Mexico state of Tamaulipas, near the site where suspected drug gang members massacred 72 migrants last summer, authorities said Wednesday.

Ruben Dario, a spokesman for the Tamaulipas state Attorney General’s Office, said the site was being excavated to determine the exact number of dead and their identities.

State officials said the graves were found by a military patrol, but the press offices of both the Defense Department and Mexico’s navy said they could not immediately confirm the discovery. The site is about 80 miles from the border at Brownsville, Texas.

The mass burial was discovered late Tuesday 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.

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 said occurred after the migrants refused to work for the cartel.

Mexican drug cartels have taken to recruiting migrants, common criminals and youths, Mexican authorities said. Drug gunmen also operate informal checkpoints on highways in Tamaulipas and other northern states, where they hijack cars and rob and sometimes kill drivers.

San Fernando is on a major highway that leads to the U.S. border, but it wasn’t immediately known whether the victims found in the mass grave had been kidnapped from that road.

Drug gangs across Mexico also sometimes use such sites 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.”