The four men stood at the mouth of the downtown drainage tunnel in Nogales, on the wrong side of the U.S.-Mexico border. It was just before dawn on January 30, 2015, and a recent storm had unleashed a torrent of choking flood water that roared in the darkness.

On these occasions, U.S. border agents habitually open a security gate inside the tunnel to give the rushing northbound current someplace to go, to spread its treacherous fingers across the rocky wash just over the line in Arizona and ease fatal flooding on the Mexican side.

The coyotes knew this. Here, they realized, was a chance to slip past the unblinking scrutiny of la migra and pull off a rare urban crossing of the well-guarded border.

And so they gave the order, commanding the men in their charge to take a desperate risk that made sense in the unjust and wicked way poor people must seek better lives.

“Jump,” they said.

The four were mostly strangers: two cousins and a pair of Mexican nationals they had met hours before the gambit. Jorge Agustin Corrales Zuniga, 25, was from Sinaloa. Alberto Sanchez, a gray-haired 58-year-old from Veracruz, was old enough to be the other man’s father. His U.S.-made pacemaker was malfunctioning and he was returning to America to have it examined.

The cousins both were just 18 and hailed from Oaxaca, one of Mexico’s poorest provinces. Just a week before, Ulises Castellanos Juarez and Kevin Aguilar Juarez had been caught crossing the border and were delivered back to where they had started.

So, like the countless crossers before them, they tried again. But this new attempt would require even more payoff money, pose an even greater risk.

From the start, things went wrong. No one can say whether the men were overcome by the frigid current or hit by a surprise wall of water so common in desert floods, but they were immediately separated, kidnapped by the dark and rushing torrent.

Their guides abandoned them.

“They’re cowards,” Santa Cruz County Sheriff Antonio Estrada later would say. “That’s the first thing they do — leave the scene.”

Zuniga somehow reached safety in a stairwell on the U.S. side, but the others weren’t so lucky. They were swept away, their flailing captured by a U.S. Border Patrol camera.

Hours later, six miles downstream, firefighters pulled a body from the water. It was that of Sanchez, his pacemaker useless to him now. He carried no identification and was listed as a John Doe.

A week later, a mile away in the garbage-strewn Nogales wash, a public-works employee spotted another corpse. It was Kevin. Like they did with Sanchez’s body, Border Patrol agents sent the badly decomposed remains to the Pima County Medical Examiner’s office an hour north in Tucson.

There the work would begin to identify both bodies — men with big dreams who ended up here, on a scuffed metal gurney in a coroner’s office so very far from home.

Still, Ulises was nowhere to be found; he simply had vanished.

And one woman’s search to find him eventually would become a professional obsession.

Several days later, on Wednesday, February 11, Chelsea Halstead answered the telephone inside a small, cramped office in the Pima County Medical Examiner’s facility. The low-slung buildings are in Tucson’s south industrial district, within the steady hiss of Interstate 10.

For the precise woman in her mid-20s, the call soon would set her upon a cross-cultural journey of hard work and hunches, dashed hopes, and ultimate frustration.

Halstead grew up in Flagstaff, not far from from the Navajo reservation where she learned all about empathy for the suffering poor and disenfranchised. She has long brown hair she keeps pulled back with a bobby pin as she leans in close to her computer screen.

Fluent in Spanish, she works on the front lines of the Colibri Center for Human Rights, an advocacy group whose goal is to end migrant death and rampant human suffering along the meandering, seemingly endless international border.

Since 2000, about 2,600 bodies of illegal border crossers from across Latin America have been discovered in the desert south of Tucson, an area so relentless in its hammering heat and almost unfathomable isolation that it’s become known as the Devil’s Highway — or worse, the Corridor of Death.

Each day, Colibri Center workers rummage through recovered possessions to try to match the dead with 2,400 missing-persons reports that arrive from Central America and Mexico through local consulate offices, from families, and from human rights organizations.

The work is never-ending; so far, the group has helped identify 100 found remains. But another 900 are yet to be analyzed.

In a row of orange lockers down the hall from the autopsy room, Halstead helps organize files of plastic sleeves labeled “Personal Effects” — the articles found with these bodies without names. To the practiced eye, each sleeve reveals a story: religious relics such as tattered rosaries and dog-eared cards

bearing the names of patron saints.

There also are subtle clues to the region where the person lived — snapshots of wives, children, and grandparents left behind. Often, the things they carried are heartbreaking: a child’s rendering of a galloping pony, the ultrasound image of an unborn child, or slips of paper bearing the names and numbers of relatives on the much-dreamed-about “other side,” in one case identified only as “mi mama.”

The advocacy group takes its name from the hummingbird, or colibri in Spanish, discovered in the pocket of a border crosser. For many migrants traversing the Devil’s Highway, the hummingbird is a Colibri Center workers rummage through recovered possessions to try to match the dead with 2,400 missing-persons reports that arrive from Central America and Mexico through local consulates, from families, and from human rights organizations. Facebook

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symbol of hope and safe passage.

For years now, the small nonprofit has been given space in the coroner’s office to better work with Chief Medical Examiner Gregory Hess, his researchers, and staff.

That’s where Halstead was working when she got the phone call that morning. She listened as a father from Oaxaca reported that his son and nephew had crossed the border days ago in a Nogales drainage tunnel and hadn’t been heard from since.

Halstead had heard the emotional flutter in a father’s voice so many times before — reflecting worry and a whisper of dread.

The call struck a chord for several reasons; for one, she hadn’t heard of anyone crossing the border inside the Nogales city limits; most coyotes preferred more isolated, less-guarded spots.

She also knew that, less than two weeks before, the bodies of an older man and a teenager had been recovered from an unsuccessful urban crossing in the Nogales wash.

She wasn’t sure if the events were connected — and didn’t want to offer the worried man any false hope, or worse, prematurely tragic news.

So she took a missing-person report, six pages in all, that listed information critical for use on the group’s computer database, which seeks matches between found remains and details of the missing, painstakingly joining them together like pieces of a human puzzle.

Halstead recorded each man’s age, height, weight, condition of teeth, and other identifying marks. The father said his son had recently gotten two tattoos on his chest. One read “Iker”; the other, “Cristina.” He also mentioned that his nephew had a unique physical characteristic: He was missing his left testicle.

The man talked about his son, that he was a good person.

“He is very respectful, never insults people, very amiable,” the father said. “He doesn’t drink. He’s good people. He wanted to go to the other side for his own good, to have a little bit of money, but things didn’t work out that way for him, unfortunately.”

Halstead hung up the phone. She didn’t know it then, but she’d just been contacted by Ulises’ father, Octavio Juarez.

She walked down the hall to consult with Hess, the coroner, an athletic 45-year-old who takes regular runs on his lunch hour, and Bruce Anderson, a forensic anthropologist who specializes in identifying charred and decomposed remains.

The pair already had performed autopsies on both bodies recovered from the Nogales wash. One, it turned out, was missing a left testicle.

“That’s a pretty good determinator in any nationality,” said Anderson, 60, a 16-year office veteran from Hammond, Indiana. “The number of men with one testicle is pretty low.”

The team believed it had a match. Later, researchers would compare DNA samples provided by the family through the Mexican consulate to make a more precise determination.

Anderson quotes a maxim from the reality TV show The First 48, saying “Our chances of identifying one of these remains cases is much better if we can do it right away.”

In the days to come, investigators worked on identifying Sanchez’s body, knowing they had a prime clue in their possession: the pacemaker. They did an Internet search for the device’s manufacturer, which led them to the retail outlet where the pacemaker was purchased.

They quickly narrowed the possibilities and arrived at Sanchez.

Now, Halstead hoped, she could find the missing man traveling with Kevin, whom they now knew was Ulises.

“I was really excited that we were going to be able to find him,” she recalled. “Many of these missing-person cases are needle-in-a-haystack things with tens of thousands of square miles of desert to consider. But we knew where Ulises was; he was somewhere out in that wash.”

Her plan: Contact border authorities to mobilize a search. Then, if all went well, Halstead would be able to call the father in Oaxaca and relieve one family’s anguish, assuring members that their beloved son could now receive a proper burial.

She didn’t know that the search for Ulises would become stymied by bad luck and border politics.

Her main challenge would be convincing the U.S. Border Patrol that it’s duty-bound to search for missing human beings, no matter what country they come from.

Bruce Anderson remembers that first telephone call from way back in 2001, from the relative of an undocumented migrant who had disappeared after crossing the border.

Back then, such reports still were unusual, but their frequency had begun to take on a curious and sinister uptick. Legions of poor Mexicans and Central Americans were crossing the border on foot, carrying meager possessions — a new army of desperate risk-takers.

And more of them were going missing or were turning up dead.

Most families were afraid to call U.S. law enforcement officials for fear that their relatives would be arrested and deported. So they called the medical examiner’s office instead.

In 2001, Anderson filed so-called Missing Person Report No. 1 — a document soon joined by thousands of others in the mammoth chronicling of the human exodus across Arizona’s Sonoran Desert, an unforgiving environment that soon delivered an almost daily litany of death.

The numbers of found bodies started small: less than two dozen a year through the 1980s and 1990s. But in that watershed year of 2001, the toll reached 77. The following year, the grim tally would double, to 146 bodies. Between 160 and 170 bodies were found each year for the next decade and beyond.

The worst year was 2010, when 225 remains were recovered — a rate of two bodies every three days.

Nowadays, not a week goes by that an unmarked government vehicle doesn’t arrive at the medical examiner’s office rear delivery port with more desiccated corpses, this strange fruit harvested from the Arizona desert.

“We were used to reading about migrant fatalities in the big border cities — large numbers of Mexican and Central American nationals who drowned in the Rio Grande in Texas or were hit by cars near San Diego,” Anderson said. “But we’d never seen these kind of numbers in Arizona.”

The reason, he would soon learn, was political.

In 1994, the Clinton administration introduced the controversial Operation Gatekeeper program, featuring an influx of money and manpower designed to shift the flood of illegal immigration away from the nation’s large border cities of El Paso, Texas, and San Diego.

“They were treating the U.S. border like a war zone,” Halstead said. “Their plan was to better guard the cities while leaving open vast expanses of desert.”

According to its 1994 strategy plan, the U.S. Border Patrol acknowledged that droves of migrants eventually die crossing the Sonoran Desert — sending a sobering message to those who followed that the route was too perilous.

But that never happened; the crossers kept coming, in greater and greater numbers.

One reason, Halstead said, was the North American Free Trade Agreement (NAFTA). Passed that year, the pact allowed the U.S. government to flood the Mexican market with staple crops, driving struggling south-of-the-border farmers out of business.

Said Halstead: “That’s when people began migrating north like never before."

For years after 2001, researchers at the Pima County Medical Examiner’s Office handled the onslaught of reports on the dead and missing border crossers.

But Anderson faced a conundrum: How could he keep a professional distance at the autopsy table when he might have talked to the deceased’s grieving mother just the day before?

That changed in 2006, when he received a letter from college graduate Robin Reineke. She was considering taking an advanced degree in cultural anthropology at the University of Arizona and wanted the advice of Anderson, a part-time faculty member there.

Anderson was impressed with the young woman. Later, the two met in Tucson, and Reineke chose the University of Arizona. Not long afterward, she began working at the medical examiner’s office as a volunteer graduate student.

Anderson recalls that first day, when he tossed a stack of missing-person reports on Reineke’s desk.

“I told her to take it from here,” he said. “For me, talking to grieving relatives had become too much of an emotional duty.”

Reineke soon applied for private and federal grants to streamline the record-keeping process. She found peace in the work.

“Nobody deserves to waste away in the desert,” she said. “And the families of the dead shouldn’t be left to wonder years about their fate.”

In 2010, Reineke spoke to an undergraduate class at the University of Arizona, where Halstead was a student. They struck a connection. After graduating, Halstead joined Reineke, who later founded the Colibri Center to continue building her archive of the missing.

The pair’s first project was taking part in a U.S. Department of Justice study to examine ways the bodies of migrants were collected and treated in mostly poor border counties in Texas, Arizona, New Mexico, and California.

What Halstead found troubled her. There was no unified process of handling the remains: “I thought it would be like CSI, where everyone got a full autopsy.”

Instead, in Texas, migrant bodies were disposed of in milk crates, often in mass graves, with the decision to conduct an autopsy made by an elected Justice of the Peace with little or no medical expertise.

In Imperial County, California, cremated remains were dumped in the ocean, robbing relatives of any chance of providing a family burial, Halstead said.

“When it came to handling migrant remains,” she said, “I realized that Pima County was the gold standard.”

Even though he has become weary of performing autopsies on the bodies of luckless border crossers he knows shouldn’t be dead, Anderson is proud of his office’s effort to restore their identities.

“We take the time to record the minutiae of those who die or go missing,” he said. “We keep that data on file, knowing one day a family might call with information to provide a match.”

Following Ulises’ disappearance, his family endured what Halstead calls the acute torture of ambiguous loss without answers.

She has talked to the relatives of other missing border crossers, trying to offer hope for an eventual resolution: “One step in the healing process is knowing the truth.”

Without proof their missing child is dead, some parents convince themselves that those sons and daughters are instead enduring unimaginable fates.

“One mother was convinced her son had been tortured and now was so disfigured he was afraid to reach out to her,” Halstead said.

Another was sure her child had suffered brain damage and was languishing in a mental hospital.

So for Halstead, the moment of identifying a body is bittersweet. She recalled a case where she used dental comparisons to make an initial match on the remains of a Guatemalan woman crosser in her 50s — a theory later confirmed by the medical examiner.

The outcome provided her a brief sense of elation. But she still faced informing the family that their loved one was dead.

“When I told them, they kept thanking me,” she said. “I couldn’t imagine thanking the person who delivered the news that my mother was dead.”

The positive identification of a body also relieves families of another cruelty: extortion demands from groups that had once promised their relative safe passage through the desert.

Halstead said illegal border crossings are big business for Mexican drug cartels — which seek a profit whether the border crosser makes it to the U.S. soil or not.

Often, when a migrant collapses and dies in the desert, a coyote will steal their identification. Later, the family will receive calls demanding up to thousands of dollars.

The transactions all have a foregone conclusion: Even if relatives meet the ransom by borrowing money or mortgaging their property, they eventually receive notice that the cash has not been received — and that their relative has been killed.

“Migrants always lose,” Halstead said. “And the cartels always win, no matter what.”

Halstead knew Ulises probably was dead.

There was little way he could have survived the flood waters, especially in pitch darkness. When the lethal torrent finally had receded, it left behind perverse bouquets of trash hanging from tree limbs 30 feet off the ground.

But the case taunted her: She knew the body was somewhere in the wash. It was within her reach, if she only made the effort to find it.

In late February, three weeks after the four men jumped into the drainage tunnel, Halstead e-mailed the U.S. Border Patrol in Nogales, the agency’s second-largest office — inquiring whether agents could search the wash, possibly with a cadaver dog. She included a map of where the two bodies were found.

A supervisor in the office, which is tasked with patrolling 1,100 square miles of terrain, including 32 miles along the border, said he’d have to “run it up the chain” of command. But the earliest search would be in early March, still some weeks away.

The response ruffled Halstead. In that time, the body could be pushed farther downstream, buried in sand or ravaged by predators.

“If some white hiker had gone missing,” she said, “you can bet that there would have been plenty of manpower used, including search helicopters.”

Halstead also called the Santa Cruz County Sheriff’s office, where an officer listened to her request, she said, but did not offer to organize a search.

U.S. Border Patrol officials did not return calls for comment. But Santa Cruz County Sheriff Antonio Estrada remembers the case.

Now 72, he grew up on the Mexican side of Nogales, the product of a poor family. For a half-century, he has worked in area law enforcement — 26 years with Nogales police and 23 years as sheriff: “I understand the struggles of these people who try to cross this border, about their need to come here.”

But there’s another dynamic: Santa Cruz County is the smallest — and the poorest — of all U.S. counties along the international border with Mexico. Funding always is an issue.

Estrada said his department took part in the original search in January, the day the men were washed away. But he is a practical man.

“We look for these people, no matter what country they come from,” he said. “But the time comes when we have to admit ‘We can’t do this anymore.’ We know the families want us to keep looking 24/7, but when we feel like we’re spinning our wheels, we have to play the waiting game, hoping the body will eventually turn up.”

Normally, the Colibri Center does not do missing-person searches; lacking both manpower and funding. So Halstead decided to conduct her own hunt, sending a Facebook request to friends.

On a bright Saturday in late February, she and seven others walked the rocky wash for more than four hours. She carried a list of the clothing Ulises wore on the day he disappeared: a black sweatshirt, 32-inch-waist pants, size 6 1/2 shoes, and a black backpack with a Oaxaca voter-identification card.

The urban wash was littered with trash and rotting vegetables from a nearby growing field. A few times, searchers encountered the smell of death but found only the remains of a dead deer and possum, possibly a muskrat.

Without a cadaver dog, they were amateurs playing a professional’s game. At day’s end, Halstead felt guilty. Her friends were exhausted; one was traumatized by the effort.

“It was hard work,” she recalled. “And it turned out to be unnecessary.”

Gregory Hess knows all about the crazy chances taken by illegal border crossers who traipse through the Sonoran Desert amid the heat, cold, and rattlesnakes. He has autopsied the bodies of too many who didn’t make it.

“The bodies are found under trees, under rocks, in the brush, and in the open,” he said. Some turn up just a few steps from their arranged pickup spots on the U.S. side.

The highest death toll comes in summer, when temperatures soar past 120 degrees, and as many as 60 bodies a month are pulled from the sagebrush and sand by border agents, advocacy groups, and residents.

Some remains are days old; others are scattered skeletons.

Dying from hyperthermia and heatstroke is literally hell on earth. Victims become confused and delusional. Crossers might insist to companions that they can’t go on. They drink their own urine. Some tear off clothing before collapsing.

In her office, Halstead keeps a map with red dots signifying where each body was found; the entire southern portion of Pima County is a blood-red blanket. Crossers already weak from dehydration have fallen into wells or cattle watering tanks and drowned, too exhausted to climb back out.

Other bodies are recovered with their faces imbedded with cactus spurs, after they had tried to rip open the spiky plant to get at its fluids.

Dressed in purple scrubs, Hess examined the remains of a body found in the desert. On a gurney lay numerous human bones — a pelvis, spinal cord, femur, tibia, and fibula. A mandible sits off to the side, many of its lower teeth missing.

“He’s a John,” probably an older male, Hess said. The body carried the telltale signs of the desert: bones bleached white from the sun, cracking and flaky. The remains will be footnoted and later buried in a county cemetery.

But even after the body is gone, the work remains. Colibri Center staffers catalog found items as clues for later cases. Two years ago, they created a poster of 130 types of items they have recovered from 2000 to 2013.

“The Things They Carried,” it reads. “A Memorial of Lives Lost on the Border.”

The collection includes 21 cloves of garlic, 436 crosses, one Barney stuffed toy, 37 adhesive bandages, 612 telephone calling cards, two pencil sharpeners, eight Spanish-English dictionaries, 48 limes, 32 hairbrushes, 501 prayer cards, 49 dentures, 120 pill bottles, one pager, 29 bus tickets, 64 rosaries, 38 personal letters, 21 bars of soap, 15 loaves of bread, and seven airplane tickets.

The items play a crucial role in identifying bodies often so mummified that fingerprints are impossible to obtain. Some are cultural artifacts: One migrant from Guatemala carried a cheat sheet of Mexican slang he studied to avoid detection there.

One body carried a paper with a sentence handwritten three times: “Soy de Tabasco” — “I am from Tabasco.”

Other items suggest loss. One man carried a small backpack bearing the Cars movie logo, rigged with elongated straps, suggesting he may have carried his child’s bag as a memento. Inside, investigators found several crayons.

A wrinkled letter in another crosser’s pocket was adorned by a child’s version of a pony. “Dear Papa,” it began in Spanish. “Don’t forget me. I’ll miss you. I’ll think of you every day.”

Another note drove home the personal price of the cross-desert trek. “My dear,” it read in part. “I know I’ll never see you again, but I’ll cherish our memories together. I only want the best for you.”

There were other searches for Ulises.

In late February, Halstead contacted Genevieve Schroeder, a volunteer working to find lost border crossers, and provided her with the coordinates of areas already scoured.

One day, Schroeder led eight searchers along the wash. They, too, found animal remains and towering mounds of debris wrapped tightly around trees.

But no body.

“These searches are always so frustrating and sad,” Schroeder said.

Halstead later returned to the Nogales wash, this time with her then-boyfriend. The couple walked for six hours, through wide valleys and along steep cliffs, picking their way through barbed wire and past “No Trespassing” signs.

On March 3, Halstead again e-mailed border agents. She was told that a planned search had been postponed. Six days later, she e-mailed again. This time, she was told that agents had scoured the wash but had found nothing, No other details were offered.

Halstead sent a final e-mail to the agents, asking if there were any other options for the family.

She never heard back.

A year later, Halstead carries on her work with the Colibri Center. So far, through mid-February of 2016, 11 bodies have been found in the desolate deserts of Pima County.

Halstead will work them all, but one death still sticks with her.

“He’s absolutely out there,” she said of Ulises.

Her dream is to hire 15 Colibri Center staffers to better track the dead and missing, and maintain closer contact with grieving relatives.

Meanwhile, she tries to ignore the inflamed rhetoric against illegal border crossers — hardworking people whom Republican presidential front-runner Donald Trump has characterized as rapists and thieves.

Colibri also has its share of critics in hyperconservative Arizona. Some residents celebrate the high fatality rate among border crossers.

“That will teach them to break the law,” they say.

Others want the U.S. government to catapult the bodies back into Mexico.

But ugliness comes from south of the border just as well.

In recent months, Ulises’ family has begun receiving anonymous extortion threats. Callers say the teen has been tortured and will be killed if the parents do not pay a ransom. The family refused to send money after the so-called captives would not allow them to speak to their Ulises.

Their son still is gone.

“We don’t know where he is,” Octavio Juarez said. “We are worried and desperate, more than anything. We don’t know if the cartel has him and, if he died, why hasn’t [his body] been found yet?”