Arson investigators were at the scene of a wildfire near Los Angeles that exploded in size Thursday to more than 1,700 acres and destroyed at least two homes, authorities said.

Three people had been arrested in connection with the fast-moving Colby fire, but Glendora police declined to provide further information.

L.A. County fire Inspector Scott Miller said the cause of the fire, reported shortly before 6 a.m., was still under investigation, but said arson investigators were on hand and working with Glendora police.

PHOTOS: Colby fire rages near Glendora

“Any time that we can’t immediately identify what happened, we will have them come in,” Miller said of the arson team. “Just to kind of dot our i’s and cross our t’s.”

Glendora police Lt. James De Mond confirmed three people had been arrested. He had no other information, including what the allegations against the three people were or where they were taken into custody.

“They are persons of interest in this fire,” he said.

A news conference was scheduled for 11 a.m.

Nathan Judy, a spokesman for the U.S. Forest Service, said at about 10 a.m. that more than 1,700 acres had burned.

Chris Jeffers, Glendora city manager, said in an email that the city’s emergency operations center was activated and “we have declared a state of emergency so that we can better access resources and coordinate them."

Jeffers said city officials were in the process of assessing the damage.

Inspector Tony Akins with the Los Angeles County Fire Department said the “fire is under investigation.”

About 550 fire personnel were battling the blaze, which has destroyed at least two homes.

Mandatory evacuations were in place for all residents north of Sierra Madre Avenue and east of San Gabriel Canyon Road. The fire was "spreading fairly aggressively" as winds gusted around 25 mph, fire officials said.

Los Angeles County fire Inspector Scott Miller told KTLA-TV that the fire was moving aggressively, "especially for this time of year.” He urged residents to follow evacuation orders, saying that although he understood the instinct for people to stand their ground, “their lives are way more important.”

“Act quickly,” he said. “Don’t sit around and contemplate what you should do.”

As of 9:20 a.m., L.A. County fire Inspector Keith Mora said, 550 fire personnel were fighting the fire on the ground with the assistance of eight helicopters and two super-scooper tanker planes. Multiple fire and law enforcement agencies were functioning under unified command.

Scott Sukup of the National Weather Service in Oxnard said conditions were ripe for fire because “obviously we still have the dry and windy conditions going on.”

A red flag warning was extended for that area and other parts of Los Angeles County through 3 p.m. Friday, and Sukup said gusts in the mountains are expected to be 40 to 50 miles per hour with single-digit humidity.

Currently, Sukup said, winds in the area of the fire were probably gusting to about 25 miles per hour with a temperature of 60-65 degrees.

The high for the area Thursday was expected to be in the mid-80s in lower elevations and in the 70s for the mountains.

But humidity will be in the single digits all day, Sukup said.