LAKE COUNTY, Calif. – Cal Fire law enforcement officers have determined the cause of one of the two fires that burned together to form the Rocky fire, which scorched tens of thousands of acres and burned dozens of structures after it began late last month.

A report from the Lake County Sheriff's Office said Cal Fire concluded the cause of the first of the two fires was the result of the failure of a gas water heater in an outbuilding that contained flammable liquids.

The outbuilding fire then caught the nearby vegetation on fire and quickly spread to the surrounding area, officials reported.

While the cause has been determined, the circumstances surrounding the fire starting remain under investigation, according to the report.

The cause of the second fire is still under investigation, the sheriff's office said.

The Rocky fire started on Wednesday, July 29, at 3:29 p.m. off of Morgan Valley Road east of the community of Lower Lake.

When firefighters were first dispatched to the incident, it was reported as a structure fire on Rocky Creek Road, with a second fire later reported on Morgan Valley Road, as Lake County News has reported.

The fire burned approximately 69,438 acres over the next 16 days. The last damage assessment from county officials put the number of structures destroyed at 110 – nearly 50 residences with the rest outbuildings. That number may yet grow as assessments continue.

At the peak of the fire, more than 13,000 residents were impacted by evacuations and 7,500 homes were threatened, officials said.

The fire was fully contained on Aug. 14. Patrols remain under way on the fire area.

The Rocky fire remains the largest wildland fire in California this summer.

On the afternoon of Sunday, Aug. 9, the Jerusalem fire began in Jerusalem Valley, just south of the Rocky fire.

While the Jerusalem fire burned very close to the Rocky fire – with parts of the fire perimeters joining at one point – the cause is unrelated and remains under investigation, the sheriff's office reported.

On Wednesday, the Jerusalem fire remained at 25,118 acres and 95-percent containment, Cal Fire said. That fire is expected to be fully contained this week.