HILO, Hawai'i — A runaway brushfire that has burned 9,103 acres of Big Island pastureland since Sunday morning was declared 50 percent contained yesterday afternoon, and all major roads were reopened, fire officials said.

Bulldozers continued work on firebreaks around the blaze yesterday as two county helicopters dumped water on the brushfire, Deputy Fire Chief Glen Honda said.

Authorities once again closed Mamalahoa Highway at about 9 a.m. yesterday to allow fire crews to douse smoldering areas along the highway from mile marker 16 to mile marker 13 near the intersection of Mamalahoa and Waikoloa Road.

Crews with tanker trucks and hand lines soaked "hot" areas to try to extinguish underground fire, creating a damp strip along the highway where most of the fuel has already been consumed by the fire.

"Now that sector is cold," Honda said. "There's lot of burning area inside, but that perimeter looks really good, and we're going to move on from that area.

"That was really the only threat left. If anything flared up and got over the highway, then we'd be in a world of hurt resource-wise."

Bulldozers were to continue work today on firebreaks to try to keep the flames from reaching a large pocket of brush south of the intersection of Waikoloa and Mamalahoa.

The fire is generally confined by the two major highways and lava flows to the south of the burn area, but even after three days of burning, the brushfire can continue to flare up within that area, Honda said.

"You end up with huge imus with these kiawe trees and all that kind of stuff in there, and it can burn for literally weeks within that area, so the idea is to contain it," he said.

There were no homes threatened by the fire and no injuries, but the fire was the largest since another South Kohala blaze started on Aug. 1, 2005, and raced through nearly 25,000 acres, forcing evacuations from Waikoloa Village.

Fire Chief Daryl Oliveira has said the fire may have been deliberately set. The fire started about 8:16 a.m. Sunday near the 7-mile marker of Waikoloa Road, which is one to two miles north of Waikoloa Village.

A witness reported seeing a red pickup truck speeding from the scene as the witness approached the area of the fire.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.