A spokesperson for a Texas sheriff’s department says the bodies of a woman in her early 20s, a toddler and two infants were found in an area of South Texas near the Mexican border that is a common route for migrants entering the U.S. illegally.

The bodies were found Sunday in or near Anzalduas Park, which borders the river in the city of Mission, Sgt. Frank Medrano of the Hidalgo County Sheriff’s Office told The Associated Press Monday.

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.

The McAleen Monitor newspaper reported that area is known locally as El Rincon del Diablo, or the Devil’s Corner, and is often used by migrants who cross into the U.S. between border ports of entry.

Const. Larry Gallardo told the Monitor that the terrain can be difficult to traverse.

“There’s a lot of brush,” he said. “It’s like ranchland, there’s no difference.”

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

Authorities believe the woman and children may have been dead for days before being discovered.

The names of the four have not been released, and authorities are working to determine their country of origin.

Texas Sen. John Cornyn, reacting to a post from the sheriff’s office announcing the deaths, said it was time to overhaul the country’s asylum system. Cornyn promoted a bill he’s co-sponsored on the issue with Democratic congressman Henry Cuellar, also from Texas.

“Sadly, this is what happens when infants are used as a ticket to enter the U.S. due to arcane laws,” said, Cornyn, the longtime Republican senator. “Inaction is not an option.”