A fallen Pacific Gas & Electric Co. power line caused the 152-acre Irving Fire that ignited in Samuel P. Taylor State Park west of Fairfax on Monday night, Marin County officials said Wednesday.

Marin County spokeswoman Laine Hendricks said the power line fell into the grasslands around the Ridge Trail, on the south side base of Mount Barnabe.

A PG&E spokesman said the company is working with state and Marin County fire officials “to understand more about the circumstances of this preliminary finding.”

“We are monitoring fire conditions from our Wildfire Safety Operations Center in San Francisco 24/7, to determine what, if any additional threats the fire may pose to our lines and facilities,” Matt Nauman, a spokesman for PG&E, said in a statement.

At least 19 customers in the area were still without power Wednesday, according to a PG&E outage map. The company estimates that power would be fully restored by evening.

As of Wednesday night, containment was 80 percent, according to the Marin County Fire Department. No homes have been burned.

Residents evacuated from the blaze were allowed to go home Tuesday night, authorities said.

Parts of Sir Francis Drake Boulevard and Arroyo Road that had been closed by the fire are now open.

Gwendolyn Wu is a San Francisco Chronicle staff writer. Email: gwendolyn.wu@sfchronicle.com Twitter: @gwendolynawu