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.
According to the sheriff's office, the two detainees were being transferred from Waccamaw Mental Health in Myrtle Beach and McLeod Loris Hospital in Loris to McLeod Behavioral Health in Darlington, S.C.
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. Rescue teams plucked the deputies from the top of the van. The inmates have not been recovered.
"They were trying to negotiate through fast-running water, and it just didn't work out", Richardson told the Associated Press.
"Once those orders are issued, we don't place those people in a detention centre", Thompson said. "It's come up two feet since just last night".
WMBF News reporter Patrick Lloyd was first on scene and saw a heavy emergency response presence that consisted of the Horry County Sheriff's Office, Marion Sheriff's Office, the National Guard, and many other agencies.
"Tonight's incident is a tragedy", said Horry County Sheriff Phillip Thompson. 'Just like you, we have questions we want answered'.
The coroner in neighboring Marion County confirmed the two deceased patients were female, and the South Carolina State Law Enforcement Division is investigating the incident, according to the statement.
Justin Bamberg, a state lawmaker and lawyer who has represented the families of several people injured or killed by law enforcement officers, said Wednesday he's perplexed by the decision to transport anyone in such uncertain weather conditions. Two deputies were in the van with them, though Wenton and Green were in the back.
The SCDOT added the driver of the vehicle had to go around a barrier to get stuck where they were. In March, Christopher Bennett died after eating drugs and suffering medical issues, a SLED investigation found.
Forecasters predict some rivers in the northeastern area of the state might not reach their highest levels until later this week or next week.
At the time of writing, the van is still in the water.
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