Two female mental health patients drowned in rising floodwaters while chained up in the back of a sheriff's van.

The van was carrying the detainees to a different facility in Darlington, South Carolina when it was overtaken by flood waters on Tuesday.

Two deputies were rescued but the detainees died.

The tragedy happened near the Little Pee Dee River, one of the bodies of water that state officials are watching following the heavy rains of storm Florence.

Marion County Coroner Jerry Richardson confirmed the victims were female. Their names have not been released.

Two female mental health patients drowned in rising floodwaters while chained up in the back of a sheriff's van. Pictured: Floodwaters from storm Florence

Two female mental health patients drowned in rising floodwaters while chained up in the back of a sheriff's van. Pictured: Floodwaters from storm Florence

'They were trying to negotiate through fast-running water, and it just didn't work out,' he said.

Holden said that deputies tried to get the victims out but couldn't. Rescue teams plucked the deputies from the top of the van.

Horry County Deputy Tom Fox told WPDE-TV that the victims were mental health patients being transported from one facility to another.

Holden wouldn't give further details on the victims' status, citing an ongoing state police investigation.

In a release, Horry County Sheriff Phillip Thompson said his office would cooperate with the probe.

'Tonight's incident is a tragedy,' he said. 'Just like you, we have questions we want answered.'

It comes after some prisons bosses in South Carolina refused to evacuate inmates over safety concerns.

Officials said that the van was near the Little Pee Dee River, one of the bodies of water that state officials are watching following the heavy rains of Florence. Pictured: Floodwaters in North Carolina

On Monday, the South Carolina Department of Corrections posted pictures of prisoners preparing sandbags to defend their facilities from flooding

Prisoners in South Carolina were pictured filling sandbags after officials refused to evacuate them to safety from storm Florence.

The South Carolina Department of Corrections tweeted pictures on Monday of prisoners preparing to defend their facilities from flooding.

Bryan Stirling, director of the department, chose not to move inmates despite prison evacuations in Virginia and North Carolina.

He said that evacuating thousands of convicts with buses on congested routes would have created a public safety concern.

A spokesman for the department added: 'In the past, it's been safer to leave them there.'

Among the prisons that are not being evacuated are Ridgeland, MacDougall, and Lieber Correctional Institutions.