The 152-acre Irving Fire burning at Samuel P. Taylor State Park northwest of Lagunitas in Marin County is 90 percent contained as of Thursday morning, county fire officials said.
Crews are removing or extinguishing burning material near the fire control lines and monitoring hot spots.
Marin County Fire Department officials said the cause of the fire is believed to be a power line that fell on the Ridge Trail on the south side of Mount Barnabe.
PG&E spokeswoman Deanna Contreras said Wednesday the utility company is working with Cal Fire and the county Fire Department to learn more about the preliminary finding.
The first smoke from the fire was reported between 7 and 7:30 p.m. Monday, Marin County fire spokeswoman Laine Hendricks said.