Horry County Sheriff's Department spokeswoman Brooke Holden said a sheriff's office van was carrying two "detainees" and two deputies to Darlington on Tuesday night when it was overtaken by floodwaters.
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.
Both Richardson and Thompson stressed that the women were not restrained.
"They're still under the water", Richardson told the AP on Wednesday morning.
'They were trying to negotiate through fast-running water, and it just didn't work out, ' he said. Compounding the problem, storm runoff from North Carolina - which was battered by Florence - is now flowing through the Palmetto State via the Yadkin-Pee Dee drainage basin, which terminates in Winyah Bay near Georgetown, S.C. Thom Berry with the South Carolina Law Enforcement Division said during part of the recovery effort, they tried putting boats in the water so they could get the victims out of the van.
By the time a rescue team arrived, the deputies were stranded on top of the van and it was too late for the women.
He said it's typical for deputies to transport mental-health patients, and restraints aren't used if the patients aren't a threat.
In the statement provided by police, HCSO cited two state laws saying officers are "authorized and required to take into custody and transport [the person] to the hospital designated by the certification".
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The South Carolina State Law Enforcement Division is now investigating the matter, according to the report.
The dam and roadway at Alton Lennon Drive in Boiling Spring Lakes, North Carolina is washed away.
"Once those orders are issued, we don't place those people in a detention centre", Thompson said.
"Tonight's incident is a tragedy", Thompson said. "Just like you, we have questions we want answered".
The death toll from Florence rose to 37 in three states, the AP report noted.
President Donald Trump is slated to visit North Carolina on September 19 to see the damage.
Wilmington is still mostly an island surrounded by floodwaters, and people are waiting for hours for handouts of necessities like food. Thousands of others around the state waited in shelters for the all-clear.
A spokesman for the department added: 'In the past, it's been safer to leave them there'.
After submerging North Carolina with almost one metre of rain, the storm dumped more than 16.5cm of rain in the Northeast, where it caused flash flooding.