The guards told police they were surprised by three armed men driving a white van who ordered the guards to lie on the ground, tied their hands behind their backs and then marched them into nearby woods.
The robbers then helped themselves to barrels filled with about $4.8 million in gold before making their getaway. Transvalue said its employees were not injured during the heist.
Transvalue chief executive officer Jay Rodriguez said the truck carrying the gold bars left Miami about 4 a.m. Sunday. The load was headed to Attleboro, Massachusetts, a town south of Boston nicknamed "Jewelry City" for the large number of manufacturers based there.
"At this time we are working with the authorities to try and resolve this matter," Rodrigez said. "We are offering a $50,000 reward for any information leading to the arrest of these criminals."
Though Interstate 95 is one of the busiest roads on the East Coast, the rural stretch where the robbery occurred is relatively isolated, bordered by trees and farms.
Wilson County Sheriff Calvin Woodard Jr. declined interview requests Monday and did not respond to messages from The Associated Press seeking additional information, such as what weapons the robbers used or how they managed to get the armored truck open. No detailed description of the suspects or their vehicle was released.
Shelley Lynch, spokeswoman for the Federal Bureau of Investigation's Charlotte field office, said agents are assisting the local sheriff and are working to determine whether a federal crime has been committed.
According to its website, Transvalue specializes in transporting cash, precious metals, gems and jewelry in armored trucks. The company says its shipments are insured for up to $100 million.