Marion County Coroner Jerry Richardson confirmed the victims were female.
The patients were being taken from Loris Hospital and Waccamaw Mental Health Hospital to McLeod, according to WPDE.
Stephen Flood, a 10-year veteran of the sheriff's office, and Joshua Bishop, a six-year veteran, escaped the van after it encountered floodwaters near U.S. Highway 76 and Pee Dee Island Road in Marion County.
The deputies waited on top of the van until rescuers arrived. The inmates have not been recovered. It could have been water pressure or the way the van was resting against a guard rail, he said. The vehicle, staffed by two Horry County Sheriff's Office deputies, was transporting two detainees from Conway to Darlington.
"If that road is in an area where it is a flood risk, and waters were rising, why were they driving on that road anyway?" he said. Thompson says the van submerged and the deputies weren't able to save the women from the back.
Rivers are still rising in SC and will continue throughout the week, the state's Emergency Response Team said Wednesday morning.
"They were trying to negotiate through fast-running water, and it just didn't work out", Richardson told the Associated Press.
County Sheriff Phillip Thompson said in a statement obtained by ABC News that South Carolina State Law Enforcement Division is investigating the tragic incident.More news: Nissan, Renault, Mitsubishi Will Put Google’s Android OS In Cars
Their names also yielded no records in the Horry County jail and court index systems. He said it's typical for deputies to transport mental-health patients, and restraints aren't used if the patients aren't a threat.
The van couldn't be removed Tuesday night because of rising waters and risky conditions, according to the release.
Green's mother declined to speak to a reporter Wednesday but said, "It's just a tragedy".
ABC News reports that the van was traveling on Highway 76 when it was overtaken by severe flooding.
"We want those who are responsible to be held accountable", it reads in part.
The SCDOT added the driver of the vehicle had to go around a barrier to get stuck where they were. At almost 7 p.m. Wednesday, State Law Enforcement Division spokesman Thom Berry confirmed the bodies had been recovered.
The Little Pee Dee River reached major flood stage after Hurricane Florence battered the Carolinas over the weekend and is expected to crest at 16 feet on September 21, seven feet higher than flood stage.
According to statute, the documents authorizing the admission require "a law enforcement officer, preferably in civilian clothes and preferably with crisis intervention training, to take into custody and transport the person to the hospital designated by the certification".