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.
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.
He said deputies were awaiting FBI agents to lead the investigation.
At least seven children 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.