LOS ANGELES, California (CNN) -- A fire in steep canyons and rural communities near Los Angeles, California, originated in a mulch pile, investigators have found, and they are trying to determine how it started, a fire official said Wednesday.
The fire was 20 percent contained Wednesday after burning 9,700 acres, a CalFire official said.
Robert Lewin of the California Department of Forestry and Fire Protection, or CalFire, told reporters that arson is one possible cause of the fire that is burning in the largely farming and ranching area.
The Guiberson Fire, named after the road near its point of origin in Ventura County, was blazing for a second day Wednesday and lapping at the northern edge of the town of Moorpark. The fire was 20 percent contained after scorching more than 15 square miles, or about 9,700 acres, Lewin said.
He said he hoped the fire would be contained by 8 a.m. Saturday. More firefighters were en route to join the 864 on the line, he said. Four firefighters have suffered minor smoke- and heat-related injuries.
Based on information from a reconnaissance flight, he predicted the acreage burned would increase before the day was over.
Lewin urged residents to leave their homes when asked to do so.
"Don't make it hard on the firefighters and law enforcement folks who are out there to protect the citizens," he said. "It's dangerous for us."
The fire began Tuesday morning between Moorpark and Fillmore, fire spokesmen said. Authorities ordered residents to evacuate the area between the two towns.
The fire was growing on its west edge, including the southwest area, where Moorpark College is located, Lewin said.
The public community college called off classes for Wednesday. The college told students and professors to check its Web site later in the day for updates.
The blaze threatened five high-voltage power lines that serve large areas of Ventura and Santa Barbara, CalFire said. Southern California Edison said two of the lines were "imminently threatened," according to the agency's Web site.
The flames also threatened an above-ground natural gas line and oil production fields and related facilities. Workers were ready to cut the flow in case any lines were breached, to prevent fire from spreading through them, fire spokesman Bill Nash said.
The blaze northwest of Los Angeles threatened 1,000 mostly ranch homes and 20 commercial properties, in addition to more rural areas to the west, county fire department spokesman Tom Kruschke said earlier in the day. Included in that count were 500 to 600 homes in the Bardsdale and Meridian Hills communities. The town of Somis, south of Moorpark, also was threatened.
The weather Wednesday was much like Tuesday -- hot and dry -- which worked against the efforts of firefighters. The National Weather Service predicted a high of 101, unseasonably warm, and humidity in the single digits.
Authorities estimate that the fire has cost $1.1 million to battle so far.
The Red Cross was manning an evacuation shelter at the Goebel Senior Center in Thousand Oaks, and others were available elsewhere, but the populations were reportedly slim. The Ventura County Fairgrounds was taking in large animals, and shelters were available at the Camarillo Airport for pets.