It comes after some prisons bosses in SC refused to evacuate inmates over safety concerns.
"They were trying to negotiate through fast-running water, and it just didn't work out", he told AP.
The van was carrying the detainees to a different facility in Darlington, South Carolina when it was overtaken by flood waters on Tuesday. Thom Berry with the South Carolina Law Enforcement Division said during part of the recovery effort, they tried putting boats in the water so they could get the victims out of the van.
Horry County referred all questions about the incident - including whether the women had been locked up or were merely being transported - to state investigators, who said they had no new information to release. "We are fully cooperating with the State Law Enforcement Division to support their investigation of this event".
The National Weather Service says the Lumber River rose 4 feet (1.2 meters) in 24 hours and was still rising Wednesday morning.
Richardson says the incident happened near the Little Pee Dee River, which is one of the bodies of water officials are monitoring for intense flooding following Florence.
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.
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After submerging North Carolina with almost one metre of rain, the storm dumped more than 16.5cm of rain in the Northeast, where it caused flash flooding.
Tuesday night, with the van still underwater, state police were on the way to the scene, a South Carolina Law Enforcement Division spokesperson told a local news station.
It was reported early Tuesday that the Pee Dee River was rising above flood stage and spilling its banks and roadways were flooding but some passable.
"The two deputies attempted to extricate the persons being transported", a statement from the Horry County sheriff's office noted. "Due to unsafe conditions and rising waters, additional search and rescue teams are arriving to assist in the recovery of the vehicle". But despite efforts to yank the doors open, authorities said, the floodwaters were too powerful for the deputies and the patients drowned inside.
Neither woman was handcuffed or restrained, Richardson said.
The death toll from Hurricane Florence climbed to at least 37, and North Carolina's governor pleaded with thousands of evacuees not to return home just yet.