A sharp increase in the number of people crossing into the United States through remote desert areas along the U.S.-Mexico border has officials and rights advocates worried that this summer will be especially lethal, with the potential for a spike in migrant deaths.

Much of the Biden administration’s border response in recent months has centered on caring for the unaccompanied minors who have arrived in record numbers, along with parents traveling with children. Those groups do not typically attempt to evade capture, and they usually seek out U.S. agents after crossing the border to request humanitarian protection.

Adult migrants continue to be the largest share of border crossers, however, and smuggling guides often send them through rugged desert and mountain areas where deaths from exposure rise with extreme heat. U.S. agents took more than 111,000 single adult migrants into custody in April, the highest total in more than a decade, and the number increased again in May, according to preliminary enforcement data.

“It’s going to be a brutal summer,” said Don White, a sheriff’s deputy in rural Brooks County, Tex., where hundreds of migrants have died over the past decade attempting to skirt a Border Patrol highway checkpoint by walking miles through the brush.

White said the county has recovered 34 bodies and human remains this year on the vast cattle ranches where migrants often become lost and dehydrated in 100-degree heat and harsh terrain. “I’ve never seen so many people coming through,” White said. “It’s just crazy right now.”

In southern Arizona, more migrants have been traveling north along treacherous routes typically used by drug smugglers and other hardened crossers, putting them in greater peril, U.S. Customs and Border Protection officials say. The agency is on pace to make more than 10,000 rescues during fiscal 2021, twice the number recorded in 2019 and 2020, CBP data shows.

“Many of our rescues are now pushing up to the mountain regions, which used to be exclusively narcotics traffic,” said Michael Montgomery, the head of CBP’s Air and Marine Operations, at a recent border safety event to discourage migrants from attempting the journey.

“Now we’re seeing more mixed traffic, people traveling at night, stumbling over rocks, falling over cliffs,” Montgomery said. “There is no soft place to fall in the mountains.”

CBP will have a helicopter rescue team with a paramedic on standby in Arizona for the summer, he said.

Border officials acknowledge that the increase in single adult migrants is partly driven by higher numbers of repeat or “recidivist” crossers attempting the journey. Since March 2020, authorities have relied upon Title 42 of the U.S. health code to quickly return most adult migrants to Mexico.

The Biden administration has kept the Trump-era policy in place, describing Title 42 as an essential tool to prevent the spread of the novel coronavirus in detention cells and border communities hit hard by the pandemic. But the turn-back approach has essentially removed the threat of criminal penalties or jail time, and smugglers are capitalizing by sending migrants to try again and again.

Those who successfully evade capture along the border also face lower risk of arrest once they arrive at their U.S. destinations. Arrests by U.S. Immigration and Customs Enforcement have dropped to the lowest levels on record in recent months under the Biden administration, the latest figures show.

Then there are the border crossers that agents are unable to interdict. In recent months, CBP officials have reported more than 1,000 “got away” incidents per day in which agents detect an illegal crossing without making an arrest. The expanded use of technology and sensors along the border has made it easier for CBP to know when someone enters, but officials say they don’t have enough agents to manage the volume of people arriving in recent months.

White, the sheriff’s deputy, said he sees fewer U.S. agents patrolling the ranch roads and dirt tracks through Brooks County. “Border Patrol hasn’t been in the brush as much due to processing the turn-ins,” he said, referring to the families and children who surrender to agents after crossing the Rio Grande to start the asylum-seeking process.

That means agents won’t be able to find as many migrants in distress, White said. “My guys will be carrying extra IV bags this summer for the ones we may find,” he said.

CBP does not maintain a single, centralized tally of exposure deaths and human remains recovered, because the majority of forensic services are handled by county sheriffs and coroners in border districts.

The Pima County medical examiner’s office, which is responsible for most of southern Arizona, encountered 220 remains last year, the highest in a decade. “2021 looks like it will be pretty significant as well,” Greg Hess, the county medical examiner, said in an interview. “We tend to have a bell curve over the hotter months.”

Dangerous crossings have also increased in California, where smugglers are sending migrants through rugged mountains between the Imperial Valley and San Diego, authorities say.

CBP officials have urged migrants to keep cellphone batteries charged and to try calling 911 if they become lost. The agency has placed rescue beacons and placards in Spanish and Mandarin along crossing routes, with instructions and GPS coordinates for migrants in distress who need help.

“It places a great burden on CBP to respond to remote, treacherous areas to rescue migrants placed in this perilous situation,” said Salvador Zamora, a retired Border Patrol official and former spokesman for the agency. “We have to launch massive search-and-rescue efforts, and when someone is in a deteriorating state, you’re racing against the clock to reach that location and provide lifesaving measures.”

Juanita Molina, the director of the Border Action Network, a human rights organization based in Arizona, said the economic toll of the pandemic on vulnerable populations has pushed people to attempt the dangerous journey despite the risks.

“We’re seeing people arriving who are more depleted and more desperate,” Molina said. “The ones who are crossing on foot are the poorest of the poor.”

The Trump administration added hundreds of miles of steel border barriers in the Arizona desert to deter crossings, but officials say the barriers have made little difference in terms of where they are encountering bodies or human remains.

Between 1990 and 2020, the remains of at least 3,356 migrants were recovered in southern Arizona, according to a report published in April by the University of Arizona’s Binational Migration Institute

Daniel Martinez, a sociologist who is one the report’s lead authors, said the intensification of enforcement by the Border Patrol in Arizona has “increased the lethality of migration.”

CBP strategies that redirect migrants to more remote areas are intentionally seeking to deter crossings by increasing “the rate of death and suffering along the border,” Martinez said in an interview.

CBP declined an interview request, but officials highlight their extensive rescue efforts to deflect criticism that U.S. border infrastructure is designed to be deadly.

Martinez noted that there has been a marked increase in rescues and recovery operations in the mountains north of the Lukeville, Ariz., area, one the border segments where the Trump administration added long stretches of 30-foot-barriers. Smugglers wielding demolition tools and power saws have been cutting through at particularly high rates along that span, according to agents, requiring frequent repairs.