The death toll from Storm Florence has risen to 37 after officials confirmed the deaths of two South Carolina mental health patients, officials say.
Police say they died on Tuesday when a sheriff's van transporting them was caught in floodwaters. The two officers with them were eventually rescued.
Thousands of homes and roadways in the region remain underwater less than a week after the storm made landfall.
President Donald Trump arrived in North Carolina on Wednesday to survey damage.
The latest victims in South Carolina were identified as Wendy Wenton, 45, and Nicolette Green, 43, by the Marion County Coroner, US media report.
The Horry County Sheriff's Department said in a statement that when the waters overtook the van, they rose too quickly for the pair to be saved.
A high water team eventually rescued the two deputies who were escorting them. The deputies were able to climb atop the van as waters inundated the vehicle.
"Despite persistent and ongoing efforts, floodwater rose rapidly and the deputies were unable to open the doors to reach the individuals inside the van," the police statement said, according to the Greenville News.
"At this time, the recovery effort is ongoing, and the transportation vehicle cannot be removed due to rising waters and dangerous conditions."
The detainees were women being transported from Waccamaw Mental Health and Loris Hospital to a behavioural health hospital in Darlington, South Carolina, according to the Associated Press.
"Tonight's incident is a tragedy," Sheriff Phillip Thompson told the Greenville News.
"Just like you, we have questions we want answered. We are fully co-operating with the State Law Enforcement Division to support their investigation of this event."
The tragic incident occurred near the Little Pee Dee River, which continues to rise as water from upriver North Carolina makes its way into South Carolina's waterways.
The Little Pee Dee has crested in some regions at over 46ft (14m) and its flooding has impacted town water supplies, officials say.
Florence has claimed eight lives in South Carolina thus far, and flooding is expected to worsen later this week and into next.
President Donald Trump is scheduled to arrive in North Carolina on Wednesday to survey the damage caused by Florence.
Thousands of homes are still underwater in the Carolinas and thousands of families have had to be rescued from the floods.