Colby fire

A firefighter starts a backfire with a flare gun along San Gabriel Canyon Road in Azusa as the Colby fire burns through brush in January. (Wally Skalij / Los Angeles Times / January 16, 2014)

The Colby fire that has been burning above Glendora since Thursday morning did not increase in size overnight as crews took advantage of calmer winds to reinforce containment lines, officials reported Friday.

More than 1,000 firefighters are now battling the fire, which remains 30% contained after scorching 1,709 acres, officials said. The main goal for Friday was to increase containment of the blaze, which officials said was sparked by an illegal campfire, the Los Angeles Times reports.

PHOTOS: Colby fire rages near Glendora

"The efforts last night were to strengthen the 30% containment line that was established earlier in the afternoon and to protect the fire from jumping Highway 39," Los Angeles County Fire Department Inspector Keith Mora said. "Overall, we had much success with the weather giving us the opportunity to fight aggressively and make a stand."

In addition to holding containment overnight, crews had to contend with a flare-up around 1 a.m. near Highway 39, although it never breached containment lines, officials said.

The fire broke out along Glendora Mountain Road by the Colby truck trail and was initially fueled by strong Santa Ana winds. But officials said the winds have died down a bit, which has aided their firefighting efforts.

The fire has destroyed five homes and several other structures, and forced several evacuations, one of which remained in effect for an Azusa neighborhood.

On Thursday, three men were arrested in connection with starting the fire. The men, all in their early 20s, were in the area where the Colby trail meets Glendora Mountain Road when they started a campfire, Glendora Police Chief Tim Staab said.

He said the area was not a designated camping spot and that the men tossed paper onto the fire and a gust of wind "just blew embers all over the place."

Despite the assistance of calmer winds Friday, a meteorologist with the National Weather Service said temperatures in the fire area may climb above 80 degrees, and humidity will likely be between 6% and 10%.

Mora said nine helicopters and two fixed-wing aircraft were helping to douse the flames. Bulldozers were also reinforcing and cutting containment lines around the blaze.

At a news conference Friday morning, officials asked the public to remember that conditions are still extremely dry and that fire danger remains high.

Twitter: @aribloomekatz | Facebook

ari.bloomekatz@latimes.com

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