The South Carolina Law Enforcement Division is reportedly investigating the circumstances surrounding their deaths.
Responders congregate on September 19, 2018, near where two people drowned Tuesday evening when they were trapped in a Horry County Sheriff transport van while crossing an overtopped bridge over the Little Pee Dee River, in Marion County, S.C.
Most of the deaths have occurred in North Carolina, where entire communities have been virtually cut off by floodwaters and are grappling with rivers cresting at record levels.
Thompson said his office was responding to court orders to transport the women from a hospital and a mental-health center.
The van was traveling west on USA 76 east of Nichols about 5 p.m. on Tuesday.
"If that road is in an area where it is a flood risk, and waters were rising, why were they driving on that road anyway?" he said.
They traveled northwest, on their way to Darlington about 100 miles away, across an obstacle course of blocked roads and swollen waters.
Because of darkness, responders decided trying to retrieve the women's bodies from the van Tuesday night wasn't safe.
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It was being driven by deputies from the Horry County Sheriff's Office.
Two mental health patients referred to by officials as "detainees" who drowned in the back of a sheriff's department transport van in SC have no arrest records in the state.
Authorities confirm two detainees drowned when a van was swept away in rising flood waters in SC. The Little Pee Dee River reached "major" flood stage in several spots, and forecasters believe it will continue to rise.
The two individuals being transported were confirmed dead by the Marion County Coroner. "Due to risky conditions and rising waters, additional search and rescue teams are arriving to assist in the recovery of the vehicle".
We are working hand-in-hand with the State Law Enforcement Division to cooperate in this investigation.
Richardson said the patients' bodies were being recovered Wednesday morning. Florence has claimed at least 27 lives in North Carolina.
Thompson said he wasn't sure what prevented the deputies from opening the van's rear door to save the women. In March, Christopher Bennett died after eating drugs and suffering medical issues, a SLED investigation found.