Thousands of people have been told to leave their homes as a wildfire burning Sunday in thick forest threatened rural communities in far Northern California.

The fire that sparked around 11:30 a.m. Saturday has destroyed four homes and consumed nearly 19 square miles near the towns of Manton, Shingletown and Viola, fire spokesman Daniel Berlant said. About 3,500 homes spread out across a rural area along the border of Tehama and Shasta counties are threatened as the fire continues to expand, he said.

“A good majority are immediately threatened, and a good number are in the path of the fire,” Berlant said Sunday. “We will be battling it hard today to protect as many of those homes as possible.”

The fire’s cause has not been determined, but officials said it started after a series of lightning strikes in the area.