2 mentally ill patients chained behind the sheriff's van.

Two mentally ill women chained to the back of a county sheriff's van drowned when the vehicle they were in was washed away by floods, police said.

The deputies who were transporting the two women to another South Carolina facility were rescued, local law enforcement officials told ABC News late Tuesday.

The two women have not yet been identified.

According to a statement from county sheriff Phillip Thompson, the two Horry County Sheriff's deputies who were in the van attempted to clear the patients but, due to the rapid increase in floods, they were unable to open the doors of the truck. The rescue teams responded in time to save the deputies.

The coroner for Marion's neighboring county confirmed that the two dead patients were women and that the State Law Enforcement Division of South Carolina is investigating the incident, the statement said.

"Tonight's incident is a tragedy," Thompson said in his release. "Just like you, we have questions we want to answer and we are cooperating fully with the State Law Enforcement Division to support their investigation of this event."

According to the ABC affiliate, patients were transported from the Waccamaw Mental Health Center at Loris Hospital to McLeod Health, which operates several facilities in the area.

The truck was heading west on Highway 76 in nearby Marion County when the water hit the vehicle.

The incident occurred in the Little Pee Dee River area, which extends from the Lumber River to Mullins, South Carolina. The Lumber River has overflowed its banks following the record rains recorded by Hurricane Florence.

ABC News & Louise Simpson contributed to this report.