On patrol in the harsh brush along the border in South Texas, Deputy Don White of the Brooks County Sheriff’s Office paused to study some empty water jugs, torn clothing and several indistinct footprints, looking for signs of migrants who might have been lost in the scorching heat.

Through the long summer, temperatures have lingered for days at a time at 100 degrees or higher. The heat has been stifling for many Texans, but deadly for some of those making their way through the hot, barren shrub land where migrants travel to avoid detection from Border Patrol agents.

“These are old,” Deputy White said, gesturing at the faint tracks in the dirt. “No one is in danger right now.” For now, at least, he said under his breath.

Fewer people are crossing from Mexico this year compared with last year, but already there have been more than 500 deaths in 2023 — confirmed by the discovery of bodies or partial remains by Deputy White and others like him as they conduct their grim patrols. In 2022, among the deadliest in recent years, there were 853 confirmed deaths.

Tracking migrant deaths is an imperfect science. Many drown trying to cross the Rio Grande; others succumb to sweltering desert conditions or a lack of water, with their deaths ultimately attributed to dehydration, heat stroke or hypothermia. The unrelenting heat this summer in Texas, combined with suffocating humidity, has contributed to many of the fatalities, local and federal officials said.

“It would be dangerous to be out there for several hours,” said Jeremy Katz, a meteorologist with the National Weather Service in Brownsville, Texas.

Groups like South Texas Human Rights, which tracks reports of migrants missing, detained or found dead, have seen a surge in caseloads since the spring, from 118 cases in March to double that in both July and August.

The Border Patrol has been posting warnings on social media. “Extreme temperatures contributed to 45 individuals being rescued and 10 individuals who died due to the dangerous heat and conditions,” Chief Jason Owens, the director of the agency, wrote last month on X, formerly known as Twitter. He said his agents found 13 dead migrants the previous week.

Even those migrants who manage to cross the river and make their way inland face myriad challenges, Deputy White said. Led by smugglers, many migrants take risky routes to avoid a Border Patrol checkpoint in the county seat of Falfurrias, about 80 miles north of the Rio Grande, often without enough food and water to endure the dayslong trek, he said.

America’s Southwest border has come to be known as one of the deadliest land crossings in the world.

Since 1998, at least 7,805 people have died trying to cross the border with Mexico and more than 3,527 remain missing, according to the Colibrí Center for Human Rights, an advocacy organization that reports on missing migrants and conducts DNA searches to identify remains.

Deputy White, 70, knows firsthand how unforgiving his patrol area can be: ranch land thick with cactus, thorny shrubs and occasional stands of oak and mesquite. Several hours before noon on a recent weekday morning, he and two members of his search team, John Baker, a U.S. Army veteran, and Ray Gregory, a paramedic and former Marine, were making their way under a burning sun, sweating through their uniforms and gear. Deputy White finally sat down under a tree whose shade provided little relief, trying to catch his breath.

“It doesn’t take long for someone to feel disoriented and get lost here,” he said.

Deputy White said his own views on his job were not easily defined. On the one hand, he considers himself a political conservative who wants people in places like New York and Los Angeles to understand the dangers migrants face when they do not follow the legal immigration process. But he also considers it his duty to prevent people from dying in their attempt to find a better life.

His phone never stops beeping with messages from desperate people, mostly in Latin America, whose relatives have gone missing. He pulled out his phone and read messages from a woman from Guatemala who had not heard from her brother in months. “Please find him,” she had written. She had included a photo and description: 28 years old, black eyes, brown skin and a tattoo of a rose.

For Deputy White, that was little to go on.

“Without coordinates, how can you find someone lost in this vast land?” he said.

Not every case reaches a dead end. He pulled out a photo of a man he had helped rescue in the sweltering terrain outside of Falfurrias. The man, a 44-year-old immigrant from Mexico, was so gaunt and dehydrated that he no longer looked like the man shown on the identification card he was carrying. Medics quickly gave him three liters of intravenous saline solution, which Deputy White said most likely saved his life.

The man, whose name was Arnulfo, was deported to Mexico. But he crossed illegally again, and now works at a restaurant in Mississippi. Reached by phone, Arnulfo, who did not want his last name published for fear of being deported, said he would be forever thankful to the men in uniform who had found him. “I would had died, for sure,” he said.

The bodies of those who do not survive are most often sent to local morgues, and then sometimes laid to rest in graves without names. Some are sent to labs for further examination to determine how they died.

Molly Kaplan, a doctoral researcher, works as a case manager with Operation Identification, a project of the Forensic Anthropology Center at Texas State University, analyzing the remains and personal items of dead migrants to help identify them. She said she was still moved by a case that took the researchers more than a decade to solve.

It involved Sandra Yaneth Aguilar, who was 14 in 2007 when she crossed the border near Brownsville and then vanished. After years of not knowing her whereabouts, her mother submitted a DNA sample in 2011. But it wasn’t until 2022 that investigators matched the sample to a set of remains that had been found four years earlier in an unmarked grave, along with those of dozens of other unidentified persons, in neighboring Willacy County. Sandra’s remains were ultimately released to her family, now living in New England.

“It is extremely rewarding to help give someone answers, after you go 12 years without knowing,” Ms. Kaplan said.

A lab assistant, Molly Scott, 20, said all of them had been troubled by the case. “You think, she was just 14. Where was I at 14?” she said. “I was playing on a soccer team. I was new at middle school, so I was trying to make new friends. My biggest problem at that age was learning how to drive, and this girl was immigrating on her own, trying to reach her family.”

The Texas State lab, in San Marcos, has a storage room with more than 300 human remains and dozens of bins and bags containing personal items found in the shrub lands: rosaries, prayer cards, soiled clothing. Other rooms hold fragments of skeletons — a woman struck by a truck, a man found lifeless on arid land — and researchers work there to decipher who they were.

Since the project began in 2013, the operation has received 483 remains and identified 95, including 24 this year.

In South Texas, there was a day or two of rain last week, but little real relief from the heat. “We will continue our present steamy weather for the month of September,” Deputy White lamented. “October starts the cooling trend, enough to make it easier on crossers.”

For now, he said, he will continue searching the brush for fresh footprints.