At the largest mass grave site ever found in Mexico, where 177 bodies have been pulled from deep pits, authorities say they have recovered few bullet casings and little evidence that the dead were killed with a gun.

Instead, most died of blunt force trauma to the head, and a sledgehammer found at the crime scene this month is believed to have been used in the executions, according to Mexican investigators and state officials. The search continued Sunday, with state officials warning they expect the count to rise.

They say as many as 122 of the victims were passengers dragged off buses at drug cartel roadblocks on the major highway to the United States.

The mass killings of civilians at isolated ranches 90 minutes south of the Texas border mark a new level of barbarity in Mexico’s four-year U.S.-backed drug war.

As forensic teams and Mexican marines dig through deeper and darker layers here, the buried secrets in San Fernando are challenging President Felipe Calderon’s assertions that his government is winning the war and is in control of Mexico’s cities and roads.

In the past four years, more than 35,000 people have been killed and thousands more have simply disappeared, since Calderon sent the military to battle Mexican organized crime with $1.6 billion in U.S. support. U.S. officials in Mexico worry that criminal gangs are taking over sections of the vital border region not by overwhelming firepower but sheer terror.

On Thursday, cartel gunmen sacked the city of Miguel Aleman, across the river from Roma, Texas, tossing grenades and burning down three car dealerships, an auto parts outlet, a furniture store and a gas station. Three buses were strafed with gunfire Saturday in separate attacks, wounding three people.

The U.S. State Department issued new warnings Friday advising Americans to defer nonessential travel to the entire border state of Tamaulipas and large swaths of Mexico because of the threat of armed robbery, carjacking, kidnapping and murder by organized crime.

In the red dirt tombs of San Fernando, almost all the bodies were stripped of identification, meaning no licenses, bus ticket stubs or photographs of loved ones, according to interviews with local and state officials, making the job of notifying next of kin especially difficult.

Forensic photographs shown to The Washington Post depict mummified bodies caked in dirt and badly decomposed, with signs of extreme cranial trauma. In the largest two graves, holding 43 and 45 bodies, the corpses were piled atop one another in a 10-foot-deep pit dug by a backhoe, that criminals filled over in the past four months.

The red nail polish on a young victim’s toe stands out in one photograph, along with her XS-size undergarments.

Officials in Tamaulipas say they have found 34 grave sites scattered in a wide arc around this farming town of 60,000, where Mexican marines last week established a military camp for ground and helicopter patrols.

Evidence suggests that the dead include Mexicans and Central American migrants traveling to the United States to work. Only a few of the exhumed bodies have been identified, including those of a local car salesman, a federal social worker and a Guatemalan immigrant.

Authorities have arrested 76 suspects, including alleged local Zeta boss Martin “El Kilo” Estrada, a husky, menacing figure covered in tattoos who authorities paraded before television cameras and charged as the mastermind of the homicides.

Motives for the mass killings remain a matter of speculation. “Perhaps we are seeing in the graves the results of several different confrontations and crimes committed over many months,” said Morelos Canseco Gomez, the lieutenant governor of Tamaulipas.

Canseco said authorities are still looking for an entire bus loaded with passengers that vanished on the border in March.

Savage discoveries

At least nine graves scattered around San Fernando contained only a single corpse, and some of the burial sites might hold not kidnap victims but fallen cartel comrades killed in shootouts with rivals, Canseco said.

The families of passengers taken off buses here did not receive ransom demands, investigators say, and so the victims appear not to have been killed for large sums of money, only what they might have had in their wallets and purses. The savage method of execution is also unexplained, with shuddering investigators left guessing at the mental state of the killers.

Officials say some victims may have been snatched to serve as forced recruits for the Zetas crime organization, according to five bus passengers abducted but later rescued.

San Fernando is the same place where 72 migrants from Central and South America were kidnapped and fatally shot last August, bringing condemnation from the United Nations and new focus on the perils faced by travelers crossing Mexico en route to the U.S. border.

After the massacre, Calderon sent the Mexican military to retake the town, vowing to “protect migrants and Mexican families.” But as attention on San Fernando faded, federal forces withdrew and locals say the crime gangs quickly muscled their way back in.

“People began to disappear,” said Ramon Ruiz, an apprentice priest in San Fernando. “First it was people with money, then it was anyone. They kidnapped a local farmer’s son and demanded $10,000, and when he gave them $5,000 — everything he had — they sent him half of his son.”

The criminals commandeered nearby ranches, killing the owners or driving them off, then converted barns and sheds into holding pens and execution chambers.

Silence choked the town until late last month, when state authorities received calls that large groups of bus travelers were kidnapped along the Highway 101 on March 24 and 29. Soldiers followed a tip down a maze of dirt roads out to a ranch miles off the main highway, where they freed five kidnapping victims and captured nine Zeta cell members, after killing four gunmen who were standing guard.

The suspects talked. Mexican authorities began to dig.

Hunting for loved ones

Most of the bodies recovered from San Fernando were taken to the morgue in Matamoros, across the Rio Grande from Brownsville, Texas. Families of the missing there have taped photocopied fliers about their loved ones to the walls of state forensic offices there, and more than 400 people have arrived to provide DNA samples.

“MISSING,” the fliers read: Eli Octavio Juarez, 17, last seen March 20 in a 1995 Ford Explorer with tinted windows. And Emmanuel Alejandro Zuniga, missing March 9, en route to Ciudad Victoria — “call his mama.”

Raul Lopez Zunun, a 70-year-old farmer, traveled 1,100 miles by bus from his home in southern Mexico to the forensic lab in Matamoros, clutching a photocopied picture of his son Israel Lopez. He went missing in the area in late March while en route to a job in Ohio.

“We’re looking for him in all the hospitals here,” said Lopez, who grows corn and coffee on a small farm in Chiapas. “I told him not to go.”

On Thursday, Mexican authorities arrested the police chief in San Fernando, and 16 of the department’s 25 officers are now in custody, suspected of working for the Zetas to help the gang kidnap, kill and bury their victims.

Marines patrol the streets of San Fernando, brandishing grenade launchers and heavy machine guns, but local authorities will not venture out to surrounding villages without a military escort.

In an interview, San Fernando Mayor Tomas Gloria Requena said it wasn’t true that his town was especially corrupt, or evil.

“San Fernando is Mexico,” he said. “It’s just like anywhere else.”