Two deputies were rescued but the detainees died. The women weren't identified.
Marion County Coroner Jerry Richardson told the Associated Press early Wednesday that the van was still underwater.
Recovery efforts were underway Wednesday, said Thom Berry, a spokesman for the South Carolina Law Enforcement Division. "Just like you, we have questions we want answered", he said. "We are fully cooperating with the State Law Enforcement Division to support their investigation of this event".
According to ABC affiliate WPDE, the patients were being transported from Loris Hospital Waccamaw Center for Mental Health to McLeod Health, which runs multiple facilities in the region.
According to information from SCDOT's Twitter, I-95 is being closed at mile marker 175 in both directions due to high water levels at the bridges crossing the Great Pee Dee River.
US President Donald Trump helps with flood relief supplies in North Carolina.
Gov. Roy Cooper told a news conference Tuesday is was hard for residents to leave home, and it's even harder for them to wait to go back.
The death toll from Florence rose to 37 in three states, the AP report noted.
Cooper warned that the flooding is far from over and will get worse in places.
"I know that it is a very sad situation so ive just been praying for the victims and the families of the victims", said Martin.
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'Tonight's incident is a tragedy, ' he said.
The deputies who were transporting the two women to a different facility in SC were saved, local law enforcement officials told ABC News late Tuesday.
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. Two deputies were in the van with them, though Wenton and Green were in the back.
Holden says the deputies tried to get the detainees out but couldn't open the doors.
The sheriff's deputies survived after rescue teams arrived on the scene and brought them to safety. "They were trying to negotiate through fast-running water, and it just didn't work out", Richardson told AP about the incident.
The two individuals being transported were confirmed dead by the Marion County Coroner.More news: Woman accusing Kavanaugh of assault wants FBI investigation