Members of Águilas del Desierto — Eagles of the Desert — gathered before dawn one July morning in San Diego for a foray along the Mexican border. Their mission: to find the bodies of immigrants, hundreds of whom die in the desert each year.

“I know the agony of losing a loved one to the desert,” Eli Ortiz, the group’s leader, said as we set out. He found the bodies of his brother and cousin in 2009.

Most Águilas are immigrants themselves: maids, gardeners, combat veterans, construction workers. They find purpose in providing closure for families haunted by missing relatives. They’ve undertaken this gruesome task once a month for six years.

I doubted they would find anything; the desert around the Carrizo Gorge Wilderness is vast and forbidding. But their planning was meticulous. They used satellite images, GPS mapping and intelligence gleaned from Border Patrol agents and smuggling networks.

Occasionally, family members who contact Águilas on Facebook go along. Rafael Luna, 50, said his brother went missing two months ago after setting out from Jalisco in western Mexico. “He was left out here to die,” Mr. Luna said.

José Genis González, 33, is a Navy vet, and trained as an E.M.T. He crossed with his parents when he was 2. After another volunteer picked up the scent of a dead body, Mr. González rushed to what turned out to be a grisly scene.

There was a body decomposing in the desert. Careful to avoid disrupting the remains while authorities were called, volunteers used gloves to examine the scene.

Next to the body, they found a prayer card with the image of St. Peter, thought to protect migrants from harm, and a Mexican ID card bearing the name of Adrián Luna, Rafael’s brother.

I wasn’t prepared for what happened next. Mr. Luna began shouting, “My brother, my brother!” before collapsing on the desert floor. His wails echoed across the empty hinterland.  

Agents from the San Diego County medical examiner’s office recovered the body. Coroners sometimes carry out DNA testing to definitively identify remains.

The Águilas don’t always find a body. Sometimes all they find are scattered bones or a crushed skull, evidence, perhaps, of a homicide. Authorities have found so many unidentified bodies in California that they stopped burying them, opting for cheaper cremations.

Migrants perish in the desert from dehydration, hypothermia or heatstroke. Some are raped and killed by smugglers. Rafael Luna doesn’t know how his brother died, but people in Adrián’s group told him where the body could be found.

The volunteers face challenges that include snake bites and exhaustion. One volunteer collapsed from dehydration as we trekked in the punishing heat, and had to be carried out on a stretcher. Nourishment was bologna sandwiches and cans of Pepsi.