U.S. Border Patrol agents on Sunday found four bodies near the Rio Grande in southern Texas, including three small children. The dead included one toddler, two infants and an approximately 20-year-old woman, Hidalgo County Sheriff Eddie Guerra posted in a late-night tweet.

The woman and children were found in an area notorious for illegal border crossings and drug trafficking, authorities said. Their cause of death is still under investigation, as are their identities.

“It is an area very well known for immigrants crossing the river,” Sgt. Frank Medrano, a spokesman for the Hidalgo County Sheriff’s Office, told The Washington Post.

The bodies were found in the Las Palomas Wildlife Management Area southeast of Anzalduas Park, a 96-acre green space dotted with Spanish moss-draped trees and picnic tables along the river in Mission, Tex.

The Monitor newspaper in McAllen reported the bodies were located in a remote patch of land nicknamed El Rincón del Diablo, or “the Devil’s Corner.” Located on a spit where the Rio Grande loops from south to north, the area is thick with vegetation, closed to civilians and mostly owned by the federal government, Texas Monthly reported. It’s so dark at night that headlights from U.S. agents and police are often the only lights “for miles,” according to the Monitor.

“There’s a lot of brush. It’s like ranchland; there’s no difference,” Hidalgo County Constable Larry Gallardo told the Monitor.

Anzalduas Park has been a favorite spot for illegal river crossings, especially on weekends when recreational boats fill the river and it’s relatively easy to skip the short distance across from Mexico unnoticed, the Associated Press reported in 2014.

During a January visit to the area to promote his border wall project, President Trump visited the park. Flanked by Republican GOP Texas Sens. Ted Cruz and John Cornyn, Trump examined piles of narcotics seized by Border Patrol agents. The first phase of construction on Trump’s wall project has focused on nearby areas in Hidalgo County.

Much like the rest of the Rio Grande Valley, the area has been hit by a surge of undocumented immigrants in recent months, including many Central American families seeking asylum. The waves of arrivals have overwhelmed Border Patrol facilities and led to an outcry about conditions for migrants held in detention centers.

The FBI will lead an investigation into the four bodies discovered on Sunday, Guerra said. The federal agency didn’t return a message about the case late on Sunday.

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