A firefighter driving a water truck on the Nuns fire near Oakville crashed Monday morning off Oakville Grade and was killed in the wreck, according to authorities.

The crash happened in the hills above Oakville where firefighters were protecting homes from the Nuns fire. It occurred about 6:30 a.m. as the driver, who’d been delivering water to firefighters, apparently crashed through a guard rail. The vehicle dropped down a ravine, according to reports.

A Cal Fire official said the person was contracted to help with the firefight.

The man reportedly crashed near Walnut Drive, not far from Highway 29 and Oakville Grade.

Napa County Cal Fire Chief David Shew said it was unclear what caused the crash, but “fatigue is definitely a factor” as firefighters have been working long shifts.

The Nuns fire has been spreading from the Sonoma Valley side of the Mayacamas mountains down into Napa County. Sunday night fire in that area was active, according to officials.