One woman in her early twenties, a toddler, and two infants who were found dead in Texas near the U.S.-Mexico border Sunday likely died due to dehydration and heat exposure, authorities said Monday. The four were likely crossing the Rio Grande into the U.S. when they were overcome by the heat, the Associated Press reports. The four bodies were found in or near Anzalduas Park, and authorities said the victims could have been dead for several days before being found. Hidalgo County Sheriff’s Sgt. Frank Medrano said the area where the victims were found is commonly used by migrants seeking to enter the country illegally. The FBI, which is leading the investigation because the bodies were found on federal land, said: “It’s an incredibly heart-breaking situation.”