A firefighter who died early Monday was hired as a contract firefighter, said Napa County Cal Fire Chief David Shew.

The man reportedly had been driving a water tank truck on the Oakville Grade near the Mondavi Winery when he crashed. Shew said it was unclear what caused the crash, but “fatigue is definitely a factor” as firefighters have been working long shifts.

Cal Fire typically hires contracted equipment, such as water tenders and bulldozers, for large fires.