Roberto Carlos Casso Castro has been missing since December of 2011. Two days before Christmas, he called his mother, Dr. Rosalía Castro Toss, from his black Mazda. He and his partner were running errands, and yes, he told his mother, they were coming to the family holiday dinner the next day. He hung up. They both vanished.

A social studies teacher in Veracruz, Roberto Carlos is one of more than 40,000 people in Mexico who’ve disappeared since the 2006 outbreak of the country’s war on drugs. Most are victims of organized criminal groups and corrupt state authorities. They all leave behind desperate families — like Dr. Castro, who did what any parent would after the disappearance. She went searching for answers.

Over the past three years, the mothers of Solecito have unearthed more than 300 victims

Dr. Castro visited countless authorities to demand an official investigation, none of whom have solved her son’s case. She tracked down witnesses herself, who told her that a truck had cut off her son’s car on the highway, and a group of heavily armed men had taken him and his partner away. She dug into abandoned fields rumored to be body dumps, but found nothing.

Three years after her son went missing, Dr. Castro is raising Roberto’s son, helping him cope with the paralysis that comes after a disappearance. She has turned her dental office into a fundraising bazaar that sells used clothes, because the search has become her full-time job, and, like her, there are many other mothers whose children are missing and whose searches must be funded. There is Rosa Isela López — middle-aged, full cheeks, an active listener — whose son is rumored to have been shot off his motorcycle by Veracruz state police. There is Perla Marcial, thin and fidgety, who says her teenage son was arrested at work by Veracruz state police, and never seen again. These mothers are three of more than 200 people in Veracruz who created a group dedicated to searching for missing loved ones, called Colectivo Solecito. It is one of dozens of such family collectives across Mexico.

On Mother’s Day in 2016, the women were in the streets protesting their dormant government when a young man ducked into the throng and gave them a hand-drawn map — a gift from a cartel.

The map led them to some sandy dunes at the end of a potholed drive, past a middle-class neighborhood and through a cattle pasture. Nothing distinguished it from the surrounding land. Authorities had previously searched the plot, found a handful of remains, and left.

Over the past three years, the mothers of Solecito have unearthed more than 300 victims. They’re still digging. Now called Colinas de Santa Fe, the site is the largest mass grave in Mexican history.

To find the bodies, Solecito uses a shovel and an iron rod, with the rod doing most of the investigative work. Between 5 and 6 feet tall and about an inch in diameter, a rod can be thrust into the soil like a spear to feel out what’s underneath. If it goes in easy — too easy — it means there’s a disturbance below. When the rod is pulled out, it brings up some of the soil with it, and by now, Solecito members know the smell that comes from human remains. If the rod pulls up something foul, you dig.

When there isn’t a map, searchers have begun using a more high-tech tool: the personal drone. Quadcopters can cover more ground than a team on foot and, paired with sensor technology, they can pinpoint tracts of land that may have been disturbed. Once a drone identifies tampered ground, searchers test it with the rod and dig with the shovel if necessary. Drones can also serve as a lookout, since mass graves are often on land controlled by organized crime.

Drones still have limitations. They can only pinpoint where recent digging may have happened, and the sensor technology is expensive. The system only works as part of a robust investigation, which is rare in the Mexican justice system.

But the biggest limiting factor is the sheer quantity of ground to be covered — literally the entirety of Mexico. In the past eleven years, nearly 2,000 clandestine graves have been found across the country, according to an investigation by the Quinto Elemento journalism lab. Pick seven municipalities in Mexico. Statistically, one is the site of a mass grave — and those are just the graves that have been discovered.

When the members of family collectives like Solecito search for a mass grave in places like Colinas de Santa Fe, they go in groups. Often, dangerous people would prefer the bodies not be found. The sites are isolated, making the visits conspicuous, and it’s hard to say for sure whether the visitors are being watched. There is some safety in numbers.

Drones have become a crucial part of the routine. The families sail them over the land to scan for signs of human presence, like a smoldering campfire or discarded cans of food — signals that they could come under attack because a criminal group is still there.

The drone snaps continuous photos as it flies. The families call it back to review the images and, if assured that they’re alone, they send out the drone a second time. Now they’re looking for abnormalities that suggest a potential grave, or that fit an informant’s clue about the location of a burial. They pinpoint the anomalies that they’ll later return to on foot, to dig. Both flights must be done quickly, since drone batteries tend to die within a half hour.

Government officials follow the same procedure when searching for graves. And like the families, they are afraid.

“We know they’re always watching us”

In October 2018, in a café in the shadow of the Palacio de Bellas Artes in Mexico City, a public official who participates in searches, and requested anonymity — for his safety — described the problem.

Organized crime is deeply entwined with the Mexican government. Cartels can include police, businessmen, politicians, and their appointees. The caricature of the narco — a furtive actor engaged exclusively in crime — describes but a fraction of the country’s organized crime. Even officials don’t know who to trust, he said.

When the official joins with other agencies to search the isolated fields for graves, any member of the group could actually be there on behalf of a cartel. The infiltrator might throw the other officials off the trail, or worse, out the investigation to the cartel and the other officials could be attacked in vengeance.

Officials aren’t even safe in their offices. The first investigator in Dr. Castro’s case received a call instructing him not to look into this one, to let it go. The investigator told her that he had no choice but to obey.

“We know they’re always watching us,” said the official in the café.

“They” are the organized criminal groups that have exploded in size and power since the beginning of Mexico’s drug war, which began in 2006 and, funded by at least $2.9 billion from the United States, has spawned one of the bloodiest periods in the country’s history. Tens of thousands have been killed by state security forces and cartels — the US Congressional Research Service estimates at least 150,000. Under the guise of fighting traffickers, police and military commit kidnappings, torture, and murder, while the power of the cartels only grows.

Many war victims are kidnapped and never heard from again. Sometimes their remains surface in clandestine mass graves like Colinas de Santa Fe. This fate is particularly visible since the country’s most brazen disappearance, the 2014 kidnapping of 43 students by police in Ayotzinapa. When authorities went searching for the Ayotzinapa missing they found a string of mass graves — none of which were the students. Who were all of these buried people?

Who were all of these buried people?

It’s hard to conduct a quality investigation when powerful people don’t want the truth known. Evidence can go missing and tips ignored. Officials can simply refuse to do their jobs. Even well-meaning investigators face logistical limitations. Not all public offices have a drone; often it’s only the state police force that does, so the attorney general or public prosecutor’s offices must hope they can borrow one from the police on search days. And many of those drones are archaic, sluggish, and undependable. Then there is money: sensor technology is expensive. The public finances available for investigative work are so spare that the official said he had been funding his own flights and hotels to search potential gravesites nationwide, because his bosses told him they were out of money. They promised eventual reimbursement. Then four months passed.

The last time that Fernando Ocegueda saw his son was in 2007, when a group of masked men in law enforcement uniforms stormed his Tijuana home. His son, also named Fernando, was a 23-year-old engineering student. Fernando the father was a lawyer, but the disappearance turned him into a one-man machine hell-bent on answers.

“If there has been progress [in investigations], it’s because of pressure that we, as parents, have exerted on the authorities,” he once said. He has repeatedly received death threats for his searching. “I’ve become stronger. I’m not scared of anything,” he said.

Ocegueda was one of the first people in the country to put drones to use in searching for graves

Ocegueda was one of the first people in the country to put drones to use in the search, and he later taught Tijuana government agencies to use them.

His inaugural experience with a drone was in a 20-hectare field, where he and the family collective he helped start — the United Association for the Disappeared of Baja California — had spent nearly a month digging and found no leads. “The idea to buy a drone came from a woman [volunteer] who had seen tutorials online. We bought one because it seemed like a good idea,” said Ocegueda. It was. They sent the drone’s aerial photos to an informant in prison, a member of organized crime, who pointed to the spot where his group had buried victims. They returned to dig.

They didn’t find Ocegueda’s son, but they did find other people’s children.

As stories like Ocegueda’s pile up, more groups have gotten wrapped up in solving disappearances. The Institute for Anthropological Investigations at the National Autonomous University of Mexico (UNAM) was created to study ancient civilizations, but with a sophisticated drone setup, it was hard to stay on the sidelines.

The Institute team had used drones since 2013, flying over sites where entire societies once stood. Advanced software would stitch the photos together on a university computer, a puzzle of ghostly shapes that became one composite image, the infrastructural remains of a bygone world.

Like everyone else, the UNAM team operates on a limited budget. But they have a small stable of drones — six, all DJI, mostly Phantoms — to which they affixed Canon cameras. They modified the cameras using the software Magic Lantern, enabling them to override factory settings to snap constant photos while in flight. The team also installed various lenses on the cameras to capture ultraviolet and infrared spectrums, revealing heat below ground and details in the topsoil invisible to the naked human eye.

The movement is also growing in expertise. Civil society groups have started teaching families basic investigation skills: which government branches should be involved at each stage, the proper chain of custody of evidence, the scientific disciplines that can shed light in forensics, and how technology — like drones with remote sensing capabilities — should be used.

Since November 2017, when actors like Ocegueda or the UNAM team help with searches and trainings, they are officially recognized by the Mexican government as independent experts. Their legitimate, legal participation on behalf of the disappeared became possible with the passage of the General Law on Forced Disappearance of Persons. The regulation was written by families and their civil society allies, passed by a government under pressure for its apparent inability to solve the problem alone. The law also legalizes the families’ participation in the state searches — an apt change, since it’s often the families who find the graves in the first place.

A hope is to somehow heal the chasm between citizen and state. “The situation will begin to mend itself only when there is a horizontal conversation between the government and civil society,” said Roxana Enríquez Farias, of the Mexican Forensic Anthropology Team. “But to achieve that, it’s necessary for both sides to speak the same language.”

Nowhere is that fluency more crucial than at the graves like Colinas de Santa Fe. The simple topography of sandy dunes belies what the site actually is: complex and burdened, both a final resting place of the beloved disappeared and a crime scene laden with evidence. When families forced their way in to find closure, they entered a legally amorphous position of potentially tampering with evidence, even as the state did nothing.

Sitting in Roberto Carlos’ living room in Veracruz, Rosa Isela López says that the mothers of Colectivo Solecito don’t yet trust drones, because they don’t trust the government that owns them. I ask who she does trust, if not police or public officials. She smiled and gestured toward the other mothers sitting around her. “I trust us,” she said.

Miguel Ángel León Carmona contributed additional reporting for this piece.