The brothers-in-law knew all too well that crossing the desert leading to the United States could be lethal. One had lost a father on the journey in 1995 and an uncle in 2004. And the two young men had already tried to make the trek just a few months before but surrendered to border patrol agents in exhaustion.

Yet Juan Lorenzo Luna and Armando Reyes set off again in August 2016 from their small northern Mexico town of Gomez Palacio.

Of the five who left Gomez Palacio together, two men made it to safety, and one turned back. The only information he gave was that the brothers-in-law had stopped walking and planned to surrender again. That is the last that is known of them.

As people worldwide flee war, hunger and a lack of jobs, global migration has soared to record highs, with more than 258 million international migrants in 2017. Far less visible, however, has been the toll of this mass migration — the tens of thousands of people who die or simply disappear during their journeys, never to be seen again.

In most cases, nobody is keeping track: Barely counted in life, these people don't register in death, as if they never lived at all.

At least 3,861 migrants are dead and missing on the route from Mexico to the United States since 2014, the AP found. The AP's tally includes records from the Colibri Center for Human Rights on the U.S. side and the Argentine Forensic Anthropology team in Mexico, as well as numbers from the U.N.'s International Organization for Migration and U.S. Border Patrol.

The painstaking work of identification can take years, hampered by a lack of resources, official records and coordination between countries — and even between states. The political tide has also turned against migrants, dead or alive, with the U.S. government cracking down severely on caravans of Central Americans making their way to its borders.

In the case of Luna and Reyes, officials told the families that they had scoured prisons and detention centers, but there was no sign of the missing men. Cesaria Orona even consulted a fortune teller about her missing son, Armando, and was told he had died in the desert.

One weekend in June 2017, volunteers found eight bodies next to a military area of the Arizona desert and posted the images online in the hopes of finding family. Maria Elena Luna came across a Facebook photo of a decaying body found in an arid landscape dotted with cactus and shrubs, lying face-up with one leg bent outward. There was something horribly familiar about the pose.

"That's how my brother used to sleep," she whispered.

Along with the bodies, the volunteers found a credential of a boy from Guatemala, a photo and a piece of paper with a number written on it. The photo was of Juan Lorenzo Luna, and the number on the paper was for cousins of the family. But investigators warned that while a wallet or credential was a clue, it could also have been stolen, as migrants are frequently robbed.

"We all cried," Maria Elena Luna recalled. "But I said, we cannot be sure until we have the DNA test. Let's wait."

In 2010 the Argentine forensic team and the local morgue in Pima County, Ariz., began to organize efforts to put names to the anonymous bodies found on both sides of the border. The initiative, the "Border Project," has gained support from institutions in the United States, Mexico and Central America, and to date has identified more than 183 people — a fraction of the total.

Luna and Orona gave DNA samples to the Mexican government and the Argentine group. An initial possible match for Reyes came back negative. They are still waiting.

Every time Luna hears about clandestine graves or unidentified bodies in the news, the anguish is sharp.

"Suddenly all the memories come back," she said. "I do not want to think."

Further south, the toll of the dead and the missing has been all but ignored from the single largest population movement in the world today — that of more than 2 million Venezuelans fleeing from their country's collapse.

These migrants have hopped buses across the borders, boarded flimsy boats in the Caribbean, and — when all else failed — walked for days along dirt roads and freezing mountain trails. Vulnerable to violence from drug cartels, hunger and illness that lingers even after they arrive, they have disappeared or died by the hundreds.

"They can't withstand a trip that hard, because the journey is very long," said Carlos Valdes, director of neighboring Colombia's national forensic institute. "And many times, they only eat once a day. They don't eat. And they die."

Those deaths are uncounted, as are dozens in the sea. Also uncounted are those reported missing in Colombia, Peru and Ecuador. In all the AP found at least 3,410 Venezuelans have been reported missing or dead in a migration whose dangers have gone relatively unnoticed; many of the dead perished from illnesses on the rise in Venezuela that easily would have found treatment in better times.

Among the missing is Randy Javier Gutierrez, who was walking through Colombia with a cousin and his aunt in hopes of reaching Peru to reunite with his mother.

Gutierrez's mother, Mariela Gamboa, said that a driver offered a ride to the two women, but refused to take her son. The women agreed to wait for him at the bus station in Cali, about 160 miles (257 kilometers) ahead, but he never arrived. Messages sent to his phone since that day in June have gone unread.

"I'm very worried," his mother said. "I don't even know what to do."


Christine Armario in Colombia contributed to this story.