The Horry County Sheriff's deputies reportedly tried to open the back door of the van to release the women, but were unable to because of the rising floodwater.
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.
South Carolina Highway Patrol spokesman Capt. Kelley Hughes said no trooper was stationed at the barricades, though others barricades in the area have had stationed troopers.
"It can be assumed he did" drive around a barricade, Sheriff Phillip Thompson said, adding that investigators want to know why the deputies made a decision to disregard the safety measure. "We are fully cooperating with the State Law Enforcement Division to support their investigation of this event".
The deputies were transporting Green and Newton based on court orders from the staff at Waccamaw Center for Mental Health and a mental-health center in Loris, Thompson said. The Little Pee Dee River rose almost 3 feet (1 meter) over the same time.More news: India cuts monthly crude oil loadings from Iran
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.
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.
The women's deaths bring the total to at least 35 killed since Florence made landfall Friday as a Category 1 storm.
Cooper warned locals that the flooding isn't over yet.More news: Klopp laughs off Salah's lack of goals
"They were trying to negotiate through fast-running water, and it just didn't work out", he told AP.
President Donald Trump is slated to visit North Carolina on September 19 to see the damage.
The two individuals being transported were confirmed dead by the Marion County Coroner.
Officials said the van was near the Little Pee Dee River, one of the bodies of water state officials were watching following the heavy rains of Florence. The vehicle, staffed by two Horry County Sheriff's Office deputies, was transporting two detainees from Conway to Darlington. Thompson says the van submerged and the deputies weren't able to save the women from the back. High-water rescue teams plucked the deputies from the top of the van.
The van couldn't be removed Tuesday night because of rising waters and unsafe conditions, according to the release.More news: Nokia 7.1 Plus press renders leaked ahead of launch
The van was attempting to cross a flooded area of Highway 76 in Marion County when it got swept away, according to cops.