NUEVO LAREDO, Mexico (AP) — For the investigators, the human foot -- burned, but with some fabric still attached -- was the tipoff: Until recently, this squat, ruined house was a place where bodies were ripped apart and incinerated, where the remains of some of Mexico's missing multitudes were obliterated.

How many disappeared in this cartel "extermination site" on the outskirts of Nuevo Laredo, miles from the U.S. border? After six months of work, forensic technicians still don't dare offer an estimate. In a single room, the compacted, burnt human remains and debris were nearly 2 feet deep.

Uncounted bone fragments were spread across 75,000 square feet of desert scrubland. Twisted wires, apparently used to tie the victims, lie scattered amid the scrub.

Each day, technicians place what they find -- bones, buttons, earrings, scraps of clothing -- in paper bags labeled with their contents: "Zone E, Point 53, Quadrant I. Bone fragments exposed to fire."

They are sent off to the forensic lab in the state capital Ciudad Victoria, where boxes of paper bags wait their turn along with others. They will wait a long time; there are not enough resources and too many fragments, too many missing, too many dead. 

At the Nuevo Laredo site -- to which The Associated Press was given access this month -- the insufficiency of investigations into Mexico's nearly 100,000 disappearances is painfully evident. There are 52,000 unidentified people in morgues and cemeteries, not counting places like this one, where the charred remains are measured only by weight. 

And people continue to disappear. And more remains are found.

"We take care of one case and 10 more arrive," said Oswaldo Salinas, head of the Tamaulipas state attorney general's identification team.

Meanwhile there is no progress in bringing the guilty to justice. According to recent data from Mexico's federal auditor, of more than 1,600 investigations into disappearances by authorities or cartels opened by the attorney general's office, none made it to the courts in 2020.

Still, the work goes on at Nuevo Laredo. If nothing else, there is the hope of helping even one family find closure, though that can take years. 

That's why a forensic technician smiled amid the devastation on a recent day: She had found an unburnt tooth, a treasure that might offer DNA to make an identification possible.

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When Jorge Macías, head of the Tamaulipas state search commission, and his team first came to the Nuevo Laredo site, they had to clear brush and pick up human remains over the final 100 yards just to reach the house without destroying evidence. They found a barrel tossed in a trough, shovels and an axe with traces of blood on it. Gunfire echoed in the distance. 

Nearly six months later, there are still more than 30,000 square feet of property to inspect and catalog. 

The house has been cleared, but four blackened spaces used for cremation remain. In what was the bathroom, it took the technicians three weeks to carefully excavate the compacted mass of human remains, concrete and melted tires, said Salinas, who leads work at the site. Grease streaks the walls.

Macías found the Nuevo Laredo house last August when he was looking for more than 70 people who had disappeared in the first half of the year along a stretch of highway connecting Monterrey and Nuevo Laredo, the busiest trade crossing with the United States.

The area was known as kilometer 26, a point on the highway and the invisible entrance to the kingdom of the Northeast cartel, a splinter of the Zetas. There are small shops with food and coffee. Men sell stolen gasoline and drugs. Strangers are filmed with cell phones. The power poles lining the highway farther north have been blasted with large-caliber weapons. 

Most who disappeared here were truck drivers, cabbies, but also at least one family and various U.S. citizens. About a dozen have been found alive.

Last July, Karla Quintana, head of the National Search Commission, said the disappearances appeared to be related to a dispute between the Jalisco New Generation cartel, which was trying to enter the area, and the Northeast cartel, which wanted to keep them out. It's not clear if the victims were smugglers of drugs or people, if some were abducted mistakenly or if the goal was simply to generate terror.

The phenomenon of Mexico's disappearances exploded in 2006 when the government declared war on the drug cartels. For years, the government looked the other way as violence increased and families of the missing were forced to become detectives.

It wasn't until 2018 -- the end of the last administration -- that a law passed, laying the legal foundations for the government to establish the National Search Commission. There followed local commissions in every state; protocols that separated searches from investigations, and a temporary and independent body of national and international technical experts supported by the U.N. to help clear the backlog of unidentified remains.

The official total of the missing stands at 98,356. Even without the civil wars or military dictatorships that afflicted other Latin American countries, Mexico's disappeared are exceeded in the region only by war-torn Colombia. Unlike other countries, Mexico's challenge still has no end: authorities and families search for people who disappeared in the 1960s and those who went missing today. 

President Andrés Manuel López Obrador's government was the first to recognize the extent of the problem, to talk of "extermination sites" and to mount effective searches. 

But he also promised in 2019 that authorities would have all the resources they needed. The national commission, which was supposed to have 352 employees this year, still has just 89. And Macías' state commission has 22 positions budgeted, but has only filled a dozen slots. There the issue isn't money; the difficulty is finding applicants who pass background checks.

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Disappearances are considered the perfect crime because without a body, there's no crime. And the cartels are expert at ensuring that there is no body.

"If a criminal group has total control of an area they do what we call 'kitchens,' because they feel comfortable" burning bodies openly, Macías said. "In areas that are not theirs and where the other side could easily see the smoke, they dig graves."

In 2009, at the other end of the border, a member of the Tijuana cartel confessed to having "cooked" some 300 victims in caustic lye. Eight years later, a report from a public university investigation center showed that what officially had been a jail in the border city of Piedras Negras, was actually a Zetas command center and crematorium.

Perhaps the largest such site was yet another border setting near the mouth of the Rio Grande called "the dungeon," in territory controlled by the Gulf cartel. The memory still stirs Macías. The first time he went he saw "pelvis, skulls, femurs, everything just lying there and I said to myself, 'It can't be.'"

Authorities have recovered more than 1,100 pounds of bones at the site so far. 

According to the Tamaulipas state forensic service, some 15 "extermination sites" have been found. There are also burial sites: In 2010, graves containing 191 bodies were found along one of the main migratory routes through Tamaulipas to the border. In 2014, 43 students disappeared in the southern state of Guerrero. Only three have been identified from pieces of burnt bones.

Most of the extermination sites have been found by family members who follow up leads themselves with or without the support and protection of authorities. Such search groups exist in nearly every state. 

For the families, the discoveries inspire both hope and pain.

"It brings together a lot of emotions," said a woman who has been searching for her husband since 2014 and two brothers who disappeared later. Like thousands of relatives across Mexico, she has made the search for her loved ones her life. "It makes you happy to find (a site), but at the moment you see things the way they are, you nosedive."

The woman, who requested anonymity because of safety concerns, was present for the discovery of two sites last year. When she entered the Nuevo Laredo location with Macías, she could only cry.

A few months earlier, she had found the site in central Tamaulipas where she believes her loved ones are. That day, accompanied by the state search commission and escorted by the National Guard, they entered the brush in search of a drug camp.

"I'm not well psychologically after that," she said as she showed photos of the deep graves where burnt remains were buried, some wrapped in barbed wire. They recovered around a thousand teeth, she said. 

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On a recent day in Nuevo Laredo, gloved hands sifted through the dirt, separating out bits of bone: a piece of a jaw, a skull fragment, a vertebra. 

The work is hard. The forensic technicians clear brush and then dig. Some days the temperature hovers around freezing, others it's above 100 degrees. They wear head-to-toe white protective suits and are constantly guarded. 

Security is a concern, and so authorities have separated the search function from the investigations -- the cartels appear less concerned with those just looking for bones, though anything they find could eventually become evidence in a prosecution. Each day before dusk, they are escorted to a safe house and don't leave except to return the next day to the site. 

When cartel violence exploded in Tamaulipas in 2010, the capital's morgue had space for six bodies. In a single massacre that year, a cartel killed 72 migrants. In those days, the Interamerican Commission of Human Rights denounced serious negligence in Tamaulipas's forensic work. 

Pedro Sosa, director of the state's forensic services, said that their way of working changed radically in 2018 with the establishment of the identification team. But it's not enough. "A single forensic anthropologist in the whole state is not compatible with all of this work."

It can take four months for the Nuevo Laredo remains to be cleaned, processed and arrive to the genetic lab. It can take longer if something urgent emerges like in January of last year, when nearly 20 people -- mostly migrants -- were incinerated in an attack near the border. 

Even if they manage to extract DNA, identification isn't assured because the profile will only automatically be crossed with a state database. A federal genetic database still doesn't exist. 

It could be years before even non-genetic information is added to one of the national databases. In 2020, the federal auditor said that that system had only 7,600 registered disappeared and 6,500 registered dead.

Though the federal law calls for a system in which various databases can interact, that doesn't exist, said Marlene Herbig, of the International Committee of the Red Cross. Each state or federal database of fingerprints or genetic profiles is like an island, despite calls for bridges to connect them.

No one can estimate how much money is needed or how many years it could take to see significant results in Mexico's efforts to locate and identify the disappeared. 

Herbig offered a clue: A similar effort mounted on the island of Cyprus took 10 years to identify 200 disappeared in the conflict between Greece and Turkey during the latter half of the last century. And there are many thousands more missing in Mexico than there were in Cyprus.

"This issue is a monster," Macías said. 

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AP writer Alfredo Peña in Ciudad Victoria contributed to this report.