There were no homes threatened by the fire. No injuries were reported.

The blaze — the Big Island's largest brushfire so far this year — continues to burn out of control this afternoon.

Fire crews were using four bulldozers to cut fire breaks and had four helicopters dumping water on the fire today near the intersection of Mamalahoa Highway and Waikoloa Road.

The blaze closed Mamalahoa Highway at 7:30 a.m. this morning, cutting the main commuter road between Waimea and Kailua, Kona. Mamalahoa remains closed from Kaiminani Drive to Waikoloa Road.

The fire burned 300 to 400 acres Sunday, but spread rapidly this morning as offshore winds whipped the flames through pasture land overgrown with fountain grass and other dried non-native vegetation.

Fire crews and equipment were concentrated along Mamalahoa, also known as Highway 190, between the 12- and 15-mile markers this morning in an effort to keep the fire from spreading to more pasture land north of the highway.

Fire chief Darryl Oliveira said the fire was suspicious for several reasons. It started well off the roadway, and a witness reported seeing a red pickup truck speeding up from the scene as the witness approached the area of the fire, he said.

Oliveira said there have been reports of a similar vehicle in the general area of other brushfires over the past month that are believed to be suspicious.

"We're working with police closely to try and identify any evidence, and to try and capture all the information from witnesses to try and focus their investigating," Oliveira said.