The Horry County Sheriff's Office transport van was still in the water as of about 8:45 p.m. Tuesday, WPDE reported.
Two female mental patients chained in the back of a SC county sheriff's van drowned when the... 'Just like you, we have questions we want answered'. "We are fully cooperating with the State Law Enforcement Division to support their investigation of this event".
The deputies had been scheduled to drop off the pair at nearby hospitals and were driving near the Little Pee Dee River, which had been rapidly rising due to Florence.
The Lumber River as of September 19 was still in "major" flood stage in several places, according to the National Weather Service.
Floodwaters in North Carolina were still quite high as of September 19.
The incident brings Florence's death toll to 37.
"I know for many people this feels like a nightmare that just won't end", he said.
President Donald Trump arrived in North Carolina on Wednesday to survey damage.
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Some roads in Marion and Horry County are impacted by floods after Hurricane Florence passed through the area.
In a release, Horry County Sheriff Phillip Thompson said his office would cooperate with the probe.
Horry County Sheriff's Department spokeswoman Brooke Holden said that a sheriff's office van was carrying two "detainees" and two deputies to Darlington when it was overtaken by flood waters.
Officials say the van was traveling near the Little Pee Dee River, one of the bodies of water officials in SC are watching closely as water continues to poor into the state from upriver in North Carolina following the heavy rains of Florence. The deputies were able to climb atop the van as waters inundated the vehicle.
The deputies in the van were rescued, but the patients who were chained in the back of the van died.
The van couldn't be removed Tuesday night because of rising waters and unsafe conditions, according to the release.