(CNN) South Carolina National Guardsmen allowed deputies to drive around a barricade blocking a flooded road in the aftermath of Hurricane Florence moments before two mental health patients drowned inside the vehicle, authorities say.

Two deputies escaped when high water swept away their vehicle on September 18 but they could not pull out the patients on board, the Horry County Sheriff's Office said.

Horry County spokeswoman Brooke Holden revealed on Thursday that guardsmen had waved the jail van around a barricade near the Little Pee Dee River in South Carolina. The river was over its flood stage at the time.

It's unclear why the guardsmen allowed the jail van to pass.

Lt. Col. Cindi King, the South Carolina National Guard's director of public affairs, declined to comment, citing an ongoing investigation by the South Carolina Law Enforcement Division.

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