The men accused of accidentally sparking the Colby Fire that has burned at least 1,700 acres and injured four people near Glendora could face federal charges because the blaze was believed to have been sparked on federal land, a police official said.

Glendora Police Chief Tim Staab told The Times prosecutors were expected to decide Friday whether the case would be handled by the U.S. Attorney’s Office or the Los Angeles County district attorney’s office.

Nathan Judy, a spokesman for the U.S. Forest Service, said investigators were still trying to pinpoint the exact origin of the blaze, but confirmed they also believe it began on federal ground. Judy called the federal land border in the area “sketchy,” saying that was why officials were still working to determine the precise origin.

PHOTOS: Colby fire rages near Glendora 

Glendora police have arrested three people on suspicion of recklessly starting the fire: as Clifford Eugene Henry, 22, of Glendora; Jonathan Carl Jarrell, 23, of Irwindale; and Steven Robert Aguirre, 21, a Los Angeles transient.

Staab said the men started a fire near Colby Trail and Glendora Mountain Road early Thursday because they were cold. They were allegedly tossing paper into the fire when a gust of wind “just blew embers all over the place,” Staab said.

The chief said there was “absolutely no evidence that these three men started the fire on purpose.”

“They just didn’t show very much common sense this morning in starting this camp fire,” Staab said. “Especially when it’s breezy out? Especially when it’s the driest season on record? Please.”

At last count, the blaze burning into the Angeles National Forest had damaged at least two homes – including guest houses at the historic Singer Mansion – forced mandatory evacuations, closed schools and prompted several air quality warnings across the region.

The Federal Emergency Management Agency authorized Thursday afternoon the use of federal funds to help California fight the fire. The agency said that when state officials requested the assistance, the blaze was threatening 500 homes – along with water supplies, secondary power lines and wilderness preserves – and had forced mandatory evacuations of about 7,000 people.

Four people had been injured, officials reported Thursday afternoon: two civilians and two firefighters. One of the civilians suffered what were described as minor burns, and one of the firefighters suffered a minor ankle injury. The nature of the injuries to the others were not immediately known.

Steep terrain and dry brush proved challenging for firefighters, but officials were especially wary of the winds expected to strengthen throughout the day.

Firefighters battling the Colby fire in Glendora were bracing for increasing temperatures and winds that are expected to increase through the night and into Friday morning.

The conditions add another level of difficulty for firefighters already negotiating red flag conditions.

Wind gusts atop Angeles National Forest ridges covered in flames could top out at 50 mph by 6 p.m. and last until 6 a.m. Friday, National Weather Service meteorologist Scott Sukup said.

Temperatures on Thursday were expected to peak in the mid-80s between 2 p.m. and 4 p.m. before cooling overnight as humidity climbs into double digits, Sukup said. A wind advisory is in effect around Glendora until noon Friday, he said.

Although winds at high elevations are pushing the fire southwest, cooler air from the Pomona Valley is gusting in the opposite direction, Sukup said.

Los Angeles County Fire Inspector Scott Miller said Thursday afternoon that the fire appeared to be moving northwest, into canyons deeper within the Angeles National Forest and away from many area homes.

Steep terrain and dry brush are ongoing challenges in the firefight, Miller said. But as the afternoon wears on, he added, firefighters are also concerned about hot spots that may be sparked by strengthening winds.

Also complicating efforts are winding roads in the area. Miller said crews have had to scout roads before sending firefighters ahead, delaying their work.