Two babies, a toddler and a woman were found dead near the U.S.-Mexican border, overcome by the sweltering heat in a glimpse of what could lie ahead this summer as record numbers of migrant families try to get into the United States.

Authorities believe the four may have been dead for days before the bodies were discovered on Sunday in the Rio Grande Valley. No details were released on the victims’ relationship.

It was the latest grim discovery of migrants who died while trying to cross the perilous desert and the swollen Rio Grande.

A law enforcement official close to the investigation told The Associated Press the four were overcome by the heat after fording the river. The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

Migrant families have been coming over the border in unprecedented numbers in recent months, reaching a peak in May, when 84,000 adults and children traveling together were apprehended. Nearly 500,000 immigrants have been detained at the border since the start of the year, resulting in dangerous overcrowding in U.S. holding centers.

A total of 283 migrant deaths were recorded along the 2,000-mile border last year. The death toll so far this year was not immediately released.

Three children and an adult from Honduras are believed to have died after their raft overturned on the Rio Grande in April. They had considered seeking asylum but were daunted by a long wait list to get into the U.S., according to a shelter official who met the family.

A 6-year-old immigrant from India was found dead in the triple-digit heat in Arizona this month, and seven people believed to be migrants died in June alone in irrigation canals that run alongside border barriers near El Paso. The total last year for such deaths in those canals was 11.

And the bodies of a father and nearly 2-year-old daughter from El Salvador were recovered from the Rio Grande on Monday, the Mexican newspaper La Jornada reported. The mother told the paper she watched her husband and child disappear in the strong current.

Border Patrol spokesman Ramiro Cordero said that in past years, agents would be posted near canals and hear the cries of help from migrants. But they are doing other duties this year with so many immigrants showing up, some in poor health.

“Unfortunately, because of the large influx of illegal aliens and agents having to be diverted to other duties, such as transporting, hospital escorts … there are not a lot of agents readily available to hear these cries,” he said in a statement.

The Trump administration is also under siege from critics who believe it is taking too hard a line toward humanitarian volunteers who help border crossers by leaving jugs of water in the desert and providing medical assistance.

The Justice Department prosecuted a volunteer with the aid group No More Deaths on conspiracy charges for providing two migrants with water, food and lodging last year. He faced up to 20 years in prison, but the case ended in a mistrial earlier this month when the jury deadlocked.

The immigrants who make it across the border and turn themselves in to authorities are experiencing their own problems and safety risks in government detention. Five children have died after being detained by the government since late last year, and dozens of youngsters were found last week in unsanitary conditions inside a Border Patrol station near El Paso. The government had transferred the majority of the children out of the facility by Monday.

Authorities say the weather is one factor in some of the recent deaths.

Higher-than-average snowfall in the Rocky Mountains is sending more water into the Rio Grande and adjacent canals, creating deceptively swift waters. Border agents say they are rescuing immigrants from the river on an almost daily basis.

Customs and Border Protection agents have responded to 3,330 rescue emergencies since the start of the fiscal year Oct. 1. Those numbers typically spike in the coming months as triple-digit heat becomes the norm.

During the last budget year, Customs and Border Protection rescue teams responded to more than 4,300 emergencies.

The irrigation canals near El Paso look calm on the surface and easy to cross, but their V-shape creates a quick undertow, and it is difficult to climb out of them.

“They don’t realize that once they get in there, their feet can get swept away. There’s a lot of obstacles, there’s debris in the canal, and there are headgates,” which can trap or stop people, said Capt. Kris Menendez, head of the El Paso County Water Rescue Team.

The team was training near a headgate on June 11 when one of the firefighters spotted a body in the water. By the time they got in the water, there were two bodies — one a preschool-age girl, and the other a 30-year-old man.

The names of the four who died in the Rio Grande were not immediately released, and authorities were working to determine their country of origin. The bodies were found in or near a park that borders the river in the city of Mission, Hidalgo County sheriff’s Sgt. Frank Medrano said.

The FBI is leading the investigation because the park is on federal land.

Medrano said the area is commonly used by migrants entering the country illegally.

“It’s a well-known route because it’s so close to the border,” he said.