The women, who were shackled, became trapped inside the vehicle.
Thompson planned to address reporters later Wednesday.
It had gotten dark, and responders decided it wasn't safe to try to retrieve the women's bodies from the van Tuesday night. We take a lot of pride in what we do. HCSO is conducting an internal investigation. "They're still under the water", Richardson told the Associated Press this morning. Why they chose to try to drive through is beyond me. "We will be there for whatever questions they may have, the public and anyone else".
The sheriff's press release says that the deputies tried to save the lives of Green and Newton. Or northbound motorists from Georgia could get off I-95 and take either I-26 or I-20 to Columbia and then catch I-77 to Charlotte, the agency said. She said she heard about what happened and offers the families of the victims prayers.
"I know for many people this feels like a nightmare that just won't end", he said. Richardson later told the Washington Post that sheriff's deputies often transfer mental-health patients saying, "It's a courtesy they do".
The two women have not yet been identified.
As the van traveled down USA 76 - about a half-mile from the Little Pee Dee River, which separates Mullins from the town of Nichols - high water swept away the vehicle. Two deputies were in the van with them, though Wenton and Green were in the back.
Thompson said he wasn't sure what prevented the deputies from opening the van's rear door to save the women.
The two individuals being transported were confirmed dead by the Marion County Coroner.
Investigators are in the early stages of conducting interviews and gathering evidence, SLED spokesman Thom Berry said.
"Tonight's incident is a tragedy", Thompson said in the statement. "Just like you, we have questions we want answered". "We are fully cooperating with the State Law Enforcement Division to support their investigation of this event". The pair were killed in the town of Nichols, South Carolina, along Highway 76 south of Nichols in Marion County.
The SCDOT added the driver of the vehicle had to go around a barrier to get stuck where they were.
Forecasters predict some rivers in the northeastern area of the state might not reach their highest levels until later this week or next week. It could have been water pressure or the way the van was resting against a guard rail, he said.
We are now working with the SCDNR to recover the vehicle. Rescue teams responded in time to save the deputies.
The officers involved in the incident are placed on administrative leave.