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The path across the border is littered with bodies. Bodies old and bodies young. Bodies known and bodies unknown. Bodies hidden, bodies buried, bodies lost, and bodies found. The stories of the dead haunt the frontier towns from Nuevo Laredo to Nogales, and even deep within the interior of Mexico down to Honduras, someone always knows someone who has vanished-one of los desaparecidos-during their journey north.

Many of those missing end up in the South Texas soil. Out on the Glass Ranch, a man named Wayne Johnson stumbles upon a skull, some bones, and a pair of dentures scattered near a dry pond. During a bass fishing tournament at La Amistad Lake, anglers come upon a decomposing corpse near the water's edge. Late one summer night, a train rumbles down the Union Pacific Line, but it fails to rouse a father and son slumbering on the tracks. For 2012, Brooks County, with a population of just 7,223, reported 129 deaths from immigrants trying to evade the Border Patrol checkpoint in Falfurrias, double the previous year. The county judge told the San Antonio Express-News that Brooks had run out of space for John Does in its Sacred Heart Cemetery.

The dead appear in springtime, when temperatures hit the triple digits, their fading T-shirts and tennis shoes strewn about the land like wilted wildflowers. Whether they tried to cross for money, love, or security, they did so knowing they might not make it alive. Their families keep hoping and hunting for answers-if they can. Last May, 22-year-old Aldo collapsed on a South Texas ranch and made one last, desperate cell-phone call to his older brother Alejandro in Houston. But Alejandro can't drive there to conduct a search because he, too, is here illegally. "More than anything, I would like to know what happened to my brother," he says, "because if I could retrieve some part of his body to bring down to Mexico, we could give him a proper burial."

Compared to Arizona, which identifies most of its unknown remains, Texas lets the corpses pile up. Autopsies are rarely conducted, DNA samples are not taken, and bodies are buried in poorly marked graves. Shortly after medical examiner Corinne Stern started working in Laredo, she found a 12-year-old skull from an unknown Hispanic man sitting on a shelf in the evidence room of the sheriff's office. It was devoid of any information about where it came from or how it ended up there. Mercedes Doretti of the Argentine Forensic Anthropology Team, which is working to identify the remains of missing migrants, calls the region from Houston to San Antonio and south to McAllen the "Bermuda Triangle" for bodies.

South Texas is a huge swath of ranch and farmland larger than New York state and with the population of New Mexico-about two million-most of it concentrated on the border. Urban centers have sprouted up around the international crossing points at Del Rio, Eagle Pass, Laredo, McAllen, and Brownsville, where residents are twice as likely to speak Spanish than English. Outside of these areas, the vast, vacant properties date back to Spanish land grants and have passed from their original owners to wealthy white families from Dallas or Houston, giving them a chance to play John Wayne for a weekend or shoot white-tail bucks sporting 18-point antlers. Development once amounted to hunting blinds poking out above the monotonous scrub, but the natural-gas boom has brought in trains of tractor-trailers, oil-field equipment, and scores of temporary houses with air-conditioners roaring full blast. About half of the nation's migrant deaths occur out here in the zone between the frontier towns and the U.S. Border Patrol's immigration checkpoints situated up to some 60 miles away. It's about a three-day hike through the hot, thorny scrub to evade the checkpoints.

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Although Texas law has mandated the collection of DNA from unidentified remains for the past decade and a federal grant pays for gene sequencing for any body found on U.S. soil, these programs have provided little relief for families of the missing. Just one of 28 South Texas counties has a full-fledged medical examiner's office, and that office is only a few years old. Justices of the peace, or JPs, who are elected to two-year terms, are often the highest--ranking legal officials. They may issue search and arrest warrants, decide small legal matters, and act as the coroner even if they only have a passing familiarity with law or medicine. Many JPs are first- or second-generation immigrants themselves, but they are still loath to pay $2,000 out of the county budget for an autopsy of a presumed migrant who died with no signs of foul play. Many don't even take a genetic sample, which only costs a few hundred dollars. Some JPs may be unaware of the law; others ignore it.

You often hear locals talk about the sound of a mesquite branch breaking in the night, the murmur of a foreign tongue over the hill, or a shadow dancing across their headlights. There are tales of men with sunken-in eyes, stumbling into town so parched they look like skeletons. To live in South Texas is to live among these spirits.

On a quiet street of one-story homes in Carrizo Springs, a small town that lies between Eagle Pass and Laredo, Rito Valdez tucks his red tie into the front of his dress shirt and doubles up on blue Tyvek gloves. Climbing into the bed of a pickup truck, he peels open a green body bag to examine the man's corpse inside: sun-blackened, swollen, and pulsing with pus-colored maggots that emerge from the mouth and eye sockets in rivulets. Flies dance in the June sun like quicksilver. The stench of rot blows over to me in short, hot blasts.

Valdez is a soft-spoken 32-year-old with a few extra pounds on his frame and a shiny pate as sparsely covered as the Texas chaparral. He is the third-generation director for the Memorial Funeral Chapels, a company that operates both in Eagle Pass and across the river in Piedras Negras, where his grandfather once served as mayor. Valdez has the Maverick County contract to pick up John Does for $135 apiece. He will ship bodies by road as far as Chiapas, 1,200 miles south. He also gets called out to Carrizo Springs, in the neighboring county of Dimmit, because of his Mexican connections and his walk-in freezer, a rare commodity that allows him to hold cadavers for two weeks.

Bruce Leonard, the white-haired funeral director in Carrizo Springs, is chomping on a skinny, unlit cigar as Valdez whips out his smartphone. Just yesterday, Leonard was opining to me on the good luck he'd been having as far as illegals were concerned. "Knock on wood that there haven't been any this year," he'd said. So much for that. Valdez leans over and photographs the dead man's round face, his hands, and even the logo on his blue jeans. The man had glued carpet to the soles of his sneakers to obscure his tracks in the sand, but evidently he had run out of water, or food, or energy. Border Patrol spotted his body on the Briscoe Ranch, and by the look and smell of him, he had probably been baking there for several days.

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The sheriff's department would write up a brief incident report, and a JP would sign the death certificate, but no one in the county had plans to take a DNA sample. Valdez's job is to figure out who this man is and what to do with him. If he can find the family within two weeks, usually with the help of the Mexican Consulate, he can make as much as a couple of thousand dollars. If not, he'll bury the body at a loss. "I'm not going to lie to you," Valdez says. "This is a business."

By his count, 19 of 45 bodies collected in Maverick in the last three years remain unidentified. In the local graveyard in Eagle Pass, some have white wooden crosses to mark their final resting spots. In Carrizo Springs, unknowns end up in an overgrown row at the Guadalupe #4 cemetery north of town. Their cheap aluminum markers are hidden in the tall grass, sometimes lying on their sides, bent, faded, and missing letters. One simply says: "San Pedro Ranch September 17 2011." Another reads: "Unknown Faith Ranch July 16 2010." Another: "Unidentified In Case." In Brackett-ville, about an hour north of Eagle Pass, unknown bodies are marked with printed slips of paper under a protective sheath of plastic. Some are now unreadable. "If they find a skull, they just bury it," says Diana Gonzalez of the Kinney County Treasurer's Office, which pays for pauper burials. "Arm or leg or whatever, they put it in like a bucket and come and bury it."

Leonard hands Valdez a Ziploc bag containing a wad of Mexican pesos and a photocopy of a birth certificate, which may or may not be authentic. Some migrants use fake documents to avoid being marked as two-time offenders or to avoid being deported to home countries other than Mexico, which makes an attempted return that much harder. The certificate says the man was 35. The dead are often found less than a mile from the river, Valdez says, their fatal path tracing a broad circle in the featureless terrain.

After we climb into the hearse for the 45-minute ride back to Eagle Pass, Valdez gives me some advice: "Don't breathe with your nose, just your mouth." He rolls down the windows and talks about his job. "We're not doctors," he says, "but it's almost the same. We are 24-7. People don't ask you when they can die." One moment he'll be clad in a suit jacket expressing his condolences to a family, and the next moment, he'll be hurrying through the coffin showroom and out the back door to haul home another corpse. It's been that way his whole life. He grew up on the second story of the funeral home. He started working at age 6 and has been picking up the bodies of migrants since he was 16.

Many Mexicans, he says, don't like to cremate the dead, and their families will go to great lengths to bring the body back home. In part, this stems from Catholicism-the Vatican had banned cremation until 1963-but it also speaks to the importance of funeral rites among Mexicans, who celebrate the Day of the Dead every November 2 with parades and visits to cemeteries. Valdez can't fulfill the families' wishes if the body has been outside for too long. "Sometimes they are so decomposed that it's impossible to hold the body for their family to see them," he says. "That's the worst, for people to know that they are there, but they can't see them."

Criminal gangs in Mexico have made his work more complicated. He used to make eight trips a day across the border to increasingly violent Piedras Negras, but now he goes just once a week. His drivers used to travel all night to deliver bodies to the Yucatán, but now the Zetas gang has imposed a curfew and a surcharge. When his chapel there unwittingly held the funeral for a cartel member, it was swarmed by federal agents who sequestered the mourners for questions. Once, someone called claiming he was the captain of the Zetas and had taken all Valdez's employees in Piedras hostage. If Valdez didn't start wiring him $2,000 per month, he was going to kill them one at a time. "Do it," Valdez said, hanging up his cell. He had heard about scams being orchestrated by inmates in Mexico City and secretly phoned Piedras on his second line. A funeral, he was assured, was proceeding without a hitch.

We pull up to the back of Valdez's Eagle Pass chapel, a blocky stucco building surrounded by empty lots, and I watch him load the body into his freezer and douse it with a pink formaldehyde solution. He points out two other unidentified cadavers in their late teens or early 20s. Several days earlier, they were shot in the head and left floating in their underwear in a putrid irrigation canal on the outskirts of town. They haven't been identified.

As migrants cross into South Texas, the first obstacle they encounter is the river. The Rio Grande, known as the Rio Bravo in Mexico, has its headwaters in the mountains of southern Colorado and then flows for about 1,200 miles through the Chihuahuan Desert of West Texas, the savannah-covered limestone of the Edwards Plateau, and, finally, the grapefruit groves and cotton fields of the South Texas plains. When it passes under the international bridges in Laredo, it's no wider than 50 feet across, and the emerald waters are sometimes shallow enough to wade through. At its mouth on the Gulf Coast near Brownsville, it is deep and meandering and cloudy with brown sediment.

The river became a place of death after World War II, when demand for workers outpaced the laws meant to regulate immigration. In 1942, Mexico signed a pact with the U.S., creating the first guest-worker program. But Texas was excluded because the state had not agreed to terms that established minimum wages and decent housing. Texas farmers and ranchers were happy, however, to hire those who swam across the Rio Grande illegally. An estimated 300,000 Mexicans were soon entering the U.S. each year by legal and illegal means. During the harvest season of 1949, at least one "wetback"-as the newspapers then called them-drowned each day in the Rio Grande.

Operation Wetback, the first major crackdown on illegal immigration, came in July 1954, in response to concerns about the growing immigrant population. Over several months, one million Mexicans outstaying their welcome were rounded up in neighborhoods from California to Texas and sent home by rail, bus, and ferry. Deportations by sea ended two years later after dozens of Mexicans jumped off the crowded "hell ship" Mercurio to protest its unsanitary conditions and seven drowned.

In recent decades, illegal immigration to the U.S. again soared and, with it, fatalities rose along the border. In the 1990s, the Border Patrol was catching well over a million border crossers each year. Even as arrests in the Southwest have declined in the last seven years-to just 356,873 in 2012-deaths reported by the Border Patrol have mostly remained steady at about 300 to 400 each year. With a beefed-up enforcement presence along more accessible parts of the frontier, immigrants are following riskier, more isolated trails to evade capture. Last year, the dead in Texas numbered 272, according to the Border Patrol, pushing it above Arizona's total, 186, for the first time in almost a decade.

Those counts are bound to be underestimates. Some corpses are picked up by local law enforcement, others are discovered on the Mexican side of the river, and many are never found. The best estimates for the border region as a whole come from a 2009 report by the American Civil Liberties Union and the Mexican National Commission of Human Rights, which put the dead at 600 to 700 annually. That means that every day about one body will turn up-or be lost forever-somewhere in South Texas.

It's easy to understand how so many people who cross the river run into trouble. They have enough food for a day or two but are stranded for a week. The weather is hotter or colder than they were expecting, and the water they were promised never materializes. They are unlikely to have any survival or first-aid skills. They may have come up from Guatemala or southern Mexico, and by the time they finally set out on foot in the arid Brush Country, they are out of cash, out of food, and out of good sense.

Edgar Lara can tell you what happens out there, though he'd rather not. The 30-year-old is seeing a therapist in Monterrey, Mexico, now, but he remains traumatized from the night a year ago when his 20-year-old cousin died in his arms out near El Indio, about 20 miles southeast of Eagle Pass. "Aw, man, it's hard," he tells me. He tries to speak but the words tumble out in the wrong order. "I don't talk about that to nobody," he sighs. Then, he starts again, racing through the story so that the sadness won't catch up to him.

His cousin, Yaressi Morales, had been living in Austin since she was six, when her mother took her there illegally. When Yaressi was arrested and deported, Edgar told her father he would help bring her back home. But on their first day on Texas soil, Yaressi grew so exhausted from the heat that Edgar laid her down under the shade of an elm tree. Their fellow travelers had already moved on. He tried to give her water and food, but she chewed her lips and tongue bloody as she went into convulsions from heatstroke.

Edgar was too frightened to leave her behind so he made a signal fire, sent text messages to his family for help, and yelled into the moonless sky. At half past midnight, Yaressi opened her eyes and looked up at him one last time before falling limp. He gave her CPR until sweat was burning his eyes. It was useless. He slumped back in the dirt. He cried. Then he wrapped his sweater and an old jacket tightly around her. He covered her corpse with sticks and rocks to keep wild hogs or raccoons from gnawing at it.

The vilest carcasses are the floaters. They turn green, swell up like a balloon, and stink to high heaven. In October 2010, at about ten in the morning, three Border Patrol agents in a boat found one-a man with a red plaid shirt around his waist, face down in a shallow eddy on the edge of the river. They called in the body to the sheriff's deputy in Comstock, 30 miles up the river from Del Rio, but it was in such a sorry state that it had to be buried without a name in the Sacred Heart Cemetery. Three days later, the man's brother, who works in the U.S., found out about it. The cemetery dug up the wood coffin, and the brother drove over to identify him.

The woman who owns that cemetery, Judy Cox, has Elvira eyelashes, a necklace with grape-size pearls, and a jewel-encrusted pendant ornamented with her first initial. When I meet her at the G.W. Cox funeral home one afternoon, she tells me she took over the business seven years ago when her husband died. She systematically goes through unknown plots, phoning the sheriff's department, the Mexican Consulate, and local businesses. "We don't just let it rest," she says, pulling out a typed list of a dozen names she's tracked down, both Mexicans and Americans. "I can't stand the idea of burying someone and their family not knowing what happened to them."

"There was a JP here for years," she says. "I'm not giving names, but when he was called out to a scene at a ranch on the river, he would not go any closer to the body than that doorway right there"-a distance of about 15 feet. "He would ask my husband, 'Well, what do you think happened, George?' 'Yeah, he's dead.' That's it. He didn't order any type of follow-up whatsoever."

After the dead man on the river was identified, Judy says she left him in the ground next to George. "I didn't have the nerve to tell my mother-in-law, who just turned 94, that I buried a Hispanic on our plot," she says. She even had a matching tombstone engraved for him: "José-Luis Castañeda Valdez Nov. 18, 1957–Oct. 2 2010."

Before I leave, Judy gives me a copy of José's death certificate so that I can find his mother, Aurora, just across the border in Ciudad Acuña. The next day, I park my car on the U.S. side of the crossing and take a minivan taxi over the border into the hilly streets of this relatively peaceful desert town. A group of women selling fruit juice points me to the metal gate leading to Aurora's place. Clothes are air-drying in her modest but well-tended garden. An old tree keeps the whole place shaded, and green plants sprout up from car tires and paint buckets. Aurora slowly climbs the steps. She is not even five feet tall, solid but weathered, with deep frown lines etched into her loose, earthy skin. I explain in broken Spanish that I am here to talk about her son.

"Which one?" she asks.

"The one who was lost in the-" I begin inelegantly.

"El que muríó," she replies. The one who died. Her face scrunches up and her lower lip juts out. "I have suffered so much for him," she wails. "He was so good to me."

In the guest room, she shows me the last picture of her eldest son, 52-year-old José. It's almost a mirage, a framed photo of the screen of a cheap cell phone. The words "Sprint" and "Menú" overlay the lower half of the image. Blurry and blown up beyond recognition, he stands there with a blue baseball cap and a mustache, his broad, pixelated smile stretching from cheek to cheek. José was a restless wanderer who loved the Bee Gees, spoke English like a native, and refused to settle down with any of his girlfriends. He had lived with his mother off and on over his adult life. Other times, he'd worked on ranches in Texas and at a hotel in Los Angeles where the Indian owners loved him so much, Aurora says, they joked about adopting him.

Whenever José crossed the Rio Grande, he'd call his mother promptly to let her know he was safe. When he decided in September 2010 to make the trip with his friend Alfredo and another man, he had a compass and knew a rancher who hid a key to his house and stocked it with food. It would be relatively safe. But as they neared the bluffs at the river's edge, a rattlesnake struck Alfredo. José gave him a shot of hard liquor and applied lemons and garlic to the wound-a folk treatment-so that they could make it back to Acuña to recover.

A few days later, José told Aurora he was done with crossing. He had an offer to take care of some goats in Mexico. But his friends needed him. They had never crossed themselves and begged for his help. They returned to the bluffs, but the river was higher than before. It had been four and a half feet and now it was eight. The current was swift. The three were swept downstream, and they struggled to cling to the green bamboo and reeds that line the northern shore. By the time Alfredo pulled himself to safety, José was gone. "The day that these eyes close," Aurora tells me, "is the day that I am going to rest."

There are few happy endings when someone goes missing on the border, but answers of any sort become harder to find when that person has come from southern Mexico or Central America. Fewer of these long-distance travelers have close ties to the people they are making the trek with. They use false names and false documents, and they're less likely to remain with stricken companions or to inform officials of their whereabouts. Migration has also become big business for drug cartels such as the Zetas, which control crossing points and safe houses in northeastern Mexico and have distinguished themselves through gaudy displays of cruelty, a fact that renders families fearful of making inquiries. Thousands of migrants have been kidnapped, enslaved, extorted, or killed before they even reach the border, sometimes in collusion with Mexican authorities.

The last time Anita Zelaya, an El Salvadorian, heard from her son Rafael was on May 2, 2002. "Look, Mom, we're leaving tomorrow," he told her on his final call from Frontera Hidalgo in the Mexican state of Chiapas. He was nabbed by Mexican immigration, separated from his companions, and had to hire a new coyote to give it another shot. From there, the trail ran cold. Anita spent a week investigating. She hired a guide to take her into Mexico. He taught her how to talk like a Mexican and avoid drawing attention to herself, but she left without an answer. "I want to find him alive," she says. "Whether alive or dead, an end to the uncertainty of hoping that one day he will appear, is going to calm my … my … my … my anguish, my desperation. That is what keeps us fighting. Not only to find my boy, but all of those that have vanished, right?"

Until recently, there was little hope that DNA from a body found on U.S. or Mexican soil would ever be linked to a family in El Salvador. The Federal Bureau of Investigation might work with foreign authorities on high-profile criminal cases, but most of the time it was left to migrant advocacy groups to perform low-tech detective work by going door-to-door or posting photos of the missing on bulletin boards and hoping someone recognized the person from their own journey. But a few years ago, Mercedes "Mimi" Doretti, co-founder of the Argentine Forensic Anthropology Team, which investigated human-rights abuses in the aftermath of Argentina's "Dirty War," began developing a network of forensic banks in El Salvador, Guatemala, Honduras, and Chiapas to improve the sharing of data related to missing migrants.

Doretti, now in her mid-fifties, makes frequent trips to South Texas and Central America, but since 1992 she has been based in Brooklyn, where she has a one-room office in the DUMBO neighborhood. She has a headset on over her long brown hair and is in the middle of a Skype conversation with a family in Honduras. When was the last time you spoke to him? Was he left-handed or right-handed? Does he have any dental fillings or crowns?

Doretti finished university during the twilight of Argentina's right-wing dictatorship, which was responsible for the deaths of more than 20,000 people, including political opponents and human-rights activists, in the late 1970s and early 1980s. After the country transitioned to democracy in 1983, she exhumed the mass graves of los desaparecidos and identified them, primarily using dental records, X-rays, and fingerprints. She has since achieved world recognition for her work, receiving a MacArthur Foundation "genius" award and serving as chair of the board of trustees of the United Nations Voluntary Fund for Victims of Torture. Over the past 25 years, she has worked in dozens of former war zones, including Iraqi Kurdistan, East Timor, and Bosnia, where the first large-scale effort to use DNA to identify the dead was launched. "They were making 100 identifications a month," Doretti says. "It was unbelievable."

As DNA sequencing became cheaper and more accessible in the early 2000s, the Argentineans established a nongovernmental genetic bank for relatives of the disappeared with hopes of identifying more than 600 skeletons that remained nameless. Doretti recognized that the forensic problems faced by the families of Central American migrants were not so different from those in Argentina, and she officially launched the Missing Migrants Program in August 2011. Typically, families of the missing will learn about the program through local organizations such as El Salvador's Committee of Families of Dead and Missing Migrants. Doretti's team may interview them in person in the home countries, at foreign consulates in the U.S., or over Skype. Then the team reviews medical records and takes a blood sample, which is sent for sequencing at Bode Technology in Lorton, Virginia, the company that processed remains from both Bosnia and the World Trade Center.

The Missing Migrants database now has 468 open files, each of which is linked to DNA profiles from several family members, including Anita Zelaya from El Salvador. Since 2011, Doretti has put names on 30 remains, including 12 from Texas, and has 30 promising leads pending confirmation. Some remains have dated back to 2000; most have been found after that. Doretti shows me the forensic file of one recent identification from South Texas. In that case, a family had reported their missing relative around the same time a badly decomposed body turned up. The DNA results revealed that all 15 gene alleles sequenced could be traced to either the mother or father. When a match is made, Doretti notifies the family that their relative has died and shares the evidence she has compiled in a meeting that can last several hours. She is often on the verge of tears, she says, but does her best to hold them back. "It's very clear that it's their time to cry," she says. "I should deal with my emotions on my own time."

When it comes to Texas, Doretti has no easy way to compare the DNA from families with the DNA from unidentified remains there. The National Institute of Justice awarded grants to several centers to sequence all remains found on U.S. soil, and those centers control the data. The Pima County Medical Examiner's office in Arizona is using its grant funds to perform a massive comparison of all its genetic profiles. It's a labor-intensive process, but it has been successful. By contrast, the center with grant funding in Texas, the University of North Texas Center for Human Identification, will only use the funds to evaluate a genetic match when requested by the FBI or another government agency. "It takes a lot of time to review the data," says Arthur Eisenberg, the center's director, adding that he's not sure if he's contractually allowed to devote time to reviewing the genetic profiles of foreign families. "If I do anything inappropriate, then the money may stop."

Doretti is now sequencing some Texas remains herself. With a fresh body, she has a limited amount of time to find a match before the county, which has to pay for cold storage, buries it in a pauper grave. In one case, she made a DNA match, but the body from which the DNA sample came could no longer be located in the cemetery. She is now asking Valley Forensics, a private firm that performs autopsies for Hidalgo County, to send samples from presumed migrants' bodies found in South Texas to Bode Technology for sequencing at her expense. For those long dead, however, she has little hope. "An unknown number of remains have been buried without taking any samples," she says. "It's a mess."

This spring, a tiny organization called Los Angeles del Desierto raised the alarm about the handling of bodies in Brooks County, which has unusually high numbers of deaths. The organization, based in San Diego, is run by Rafael Larraenza Hernandez, a 58-year-old Mexican native who has lived in the U.S. for 30 years. In 1996, he was moved by news stories of migrants perishing in the desert and began conducting search-and-rescue missions on the border in Arizona and California in an extended--cab pickup stocked with water, food, and first-aid supplies. Larraenza estimates that he has recovered about 35 corpses over the years.

Two years ago, he noticed an uptick in calls he was getting from families about relatives lost or in trouble in Texas. He began making the 20-hour drive out there at least once every other month, funding his trips through donations from families and the sale of homemade candles. When he visited Falfurrias, a Brooks County town 60 miles due north of McAllen, ranchers there wouldn't let him onto their properties to conduct searches. He was infuriated that of the 129 recorded deaths in 2012, 47 had been put in the ground at the Sacred Heart Cemetery without being identified by friends or relatives. "More than 50 people have asked me for help," Larraenza says. "I am pretty sure some of the missing will be in that cemetery or on those ranches."

He soon joined forces with migrant advocacy groups including the South Texas Civil Rights Project. On February 20, during a march on the county courthouse, they hand-delivered a letter to the justice of the peace, the county judge, and the county attorney, demanding that DNA samples be collected from all human remains and sent for sequencing. "Falfurrias, TX, is becoming the center of a humanitarian crisis," they wrote. "If we do not work to address this issue immediately, all indicators point to a growing trend of remains going unidentified."

Officials agreed to exhume the unknown bodies and send samples for DNA testing, as long as the funds for it didn't eat into the meager county budget. Lori Baker, a forensic archaeologist from Baylor University, a Baptist college in Waco, agreed to help. One spring day, I join Baker as she leads a dig at a Del Rio cemetery to uncover six bodies. It's almost a practice run for what she'll be doing in Falfurrias, and it's the first excavation of its kind that has ever been done in Texas.

A small woman who pulls her dirty-blond hair back into a high ponytail, she is fighting to maintain her chipper demeanor-and her energy-in the face of scorching temperatures. Helping her out are two dozen students and a former Texas Ranger everyone calls "Sarge." Two days ago, Baker was waylaid by an afternoon in the emergency room on an IV drip, coping with heat exhaustion. This morning, she nearly fainted in the shower, but now she's hunched over on black kneepads scooping up soil with a plastic dustpan.

Last decade, while working with the Pima County Medical Examiner's Office in Arizona, Baker became one of the first investigators to identify a missing migrant based on DNA from bones. It was part of a project she called Reuniting Families. "The woman I ID'd was my age and had two daughters," Baker says. "After her husband had left her, she felt the only way she could give them a better life was to go to the U.S." The woman had sold some of her land to pay for a coyote, but she twisted her ankle during the journey and was left behind by her companions. "I felt really devastated at the time," Baker says. "I was working on something that there is no good answer to." But the gratitude expressed by families when their loved ones are sent home and buried keeps her going. "The mothers say the same thing: 'Now, I have a place to pray.'"

Exhuming a corpse is a different business than entombing one. If it's fresh, then the soil is soft, the casket-if there is one-is intact, and it takes little more than a solid spade and an eager worker. As the soil compresses, the casket rots and the lid collapses. After a dozen or more years, little may be left except for metal latches, a plastic-wrapped Bible, and fragments of bones. The very stuff that once gave these bones life, the genes that allowed these people to grow and thrive, become, upon death, an eternal connection to the living. For an undocumented immigrant, it is an irrevocable identification card.

The students painstakingly map out the plots, some as old as 40 years, others as recent as 8. They sift through the debris, careful not to let a single shard escape their sieves. However admirable this excavation is, there is no shaking the sense of breaking a taboo by lifting the dead piece by piece and placing them into numbered brown paper bags. Until the gene sequencing is complete, it's impossible to know how many of the remains are actually immigrants. But it's a start.

One of the more enthusiastic students has embroidered the phrase "Them bones, them bones, them dry bones" on her blue-jean shirt for the occasion. It's a reference to an African American spiritual that has been riffed on by everyone from Fats Waller to The Kinks. The lyrics are based on the Book of Ezekiel, in which the Hebrew prophet has a vision of meeting God in the Valley of Dry Bones. "Our bones are dried, and our hope is lost," he tells God. And God responds: "When I have opened your graves, O my people, and brought you up out of your graves, and shall put my Spirit in you, and ye shall live, and I shall place you in your own land."

I lean over an open grave and see the knobby head of a femur jutting out from the dirt wall. The leg bone has been that way for days as the students work around it. One student on her knees cuts into the wall with a flat trowel, and I see the sandy soil crumble away.