(Updates size of fire, investigation into cause)
By Dan Whitcomb and Alex Dobuzinskis
LOS ANGELES, Jan 16 (Reuters) - A fast-moving wildfire that broke out on Thursday in a national forest east of Los Angeles, whipped up by winds and fueled by bone-dry brush, had damaged homes and forced the evacuation of dozens of residents, officials said.
The so-called Colby Fire broke out before dawn in the Angeles National Forest north of Glendora, about 40 miles east of downtown Los Angeles, and by mid-morning had blackened more than 1,700 acres, U.S. Forest Service spokesman Nathan Judy said.
A thick pall of orange and black smoke hung over eastern Los Angeles County and drivers were advised to keep away as residents fled the area and fire trucks sped toward the flames.
Judy said the cause of the blaze, which was burning unchecked, was under investigation, and that several people were being questioned but weren't considered suspects.
The fire had damaged at least one home, officials confirmed, as televised images showed what appeared to be several homes burned to the ground.
The flames forced the closure of Citrus College, about 2 miles (3 km) from the fire lines, Los Angeles County Fire Department spokesman Keith Mora said.
Police visited individual homes in Glendora neighborhoods near the fire to urge residents to evacuate.
Mora could not immediately provide estimates on how many residents were forced to flee.
"Early this morning, when it broke out, it burned really rapidly and it does appear like it's laying down right now," he said. "We're just trying to gain control prior to the heat-up in the afternoon."
Firefighters deployed eight helicopters and two SuperScooper airplanes to drop fire retardant on the flames.
More than 550 firefighters were involved in the effort, Mora said.
The fire was burning in steep terrain, with houses were built right up to the foot of the San Gabriel Mountains, and some isolated homes were nestled in the brush at the location of the blaze, Mora said.
"The topography is just really dangerous," he said. (Reporting by Dan Whitcomb and Alex Dobuzinskis; Editing by Cynthia Johnston, Leslie Adler and Bernadette Baum)