The federal agency said the April 3 accident in Howard occurred when a Daimler 2014 Freightliner Business Class M2 truck was carrying a three-wheel electric forklift. The truck braked during the trip, and the forklift rolled forward, broke through the cargo compartment wall and hit the gas tank behind the cab.
The forklift had been loaded with its forks pointing forward, and its parking break was applied, according to the NHTSA.
A local sheriff's department said in April that the box-style truck was slowing to turn into a driveway when the accident happened.
The truck's 59-year-old driver, Gerald Throw Sr., died in the accident, and a 29-year-old passenger was ejected and injured.
Investigators will look at the integrity of the truck's fuel system.
The truck was made by Daimler Trucks North America LLC. A spokesman said Friday morning that the company would not comment until the investigation was complete.