Border Patrol agents found four bodies, including three children, near the Rio Grande in South Texas on Sunday, Hidalgo County Sheriff Eddie Guerra said.
Two of the children were infants and one was a toddler, Guerra said. The adult was a 20-year-old woman, he said.
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The deaths come amid a huge influx of undocumented immigrants at the Southwest border and demonstrate how treacherous it is to journey, often on foot, to the U.S. from Mexico and Central America.
The bodies were found southeast of Anzalduas Park in the Las Paloma Wildlife Management Area, just north of Reynosa, Mexico, and south of McAllen, Texas, Guerra said.
A U.S. official close to the investigation said Monday that there were no early indications of foul play and they may have died from heat exposure and dehydration.
The official added that the victims appeared to be undocumented immigrants, though it wasn't clear where they were from.
More than 132,000 people were stopped while crossing the border illegally or presenting themselves at legal ports of entry in the month of May — the most in 13 years, U.S. Customs and Border Protections officials said.
At least seven children and teens are known to have died at the border since last year, though they were in CBP custody at the time. The most recent death, of 16-year-old Carlos Gregorio Hernández Vásquez, occurred last month after the teen was diagnosed with the flu.
Five of the victims have died since December. All of them were Guatemalan.