PHOENIX (Reuters) - Investigators probing the origins of Arizona's biggest wildfire on record are questioning two people in connection with an unattended campfire believed to have started the blaze, the U.S. Forest Service said on Wednesday.
The "two persons of interest" have not been publicly identified and are not under arrest, Forest Service spokesman Christopher Knopp said, declining to give further details of the investigation.
Meanwhile, ground crews battling the flames for an 18th day took advantage of another lull in high winds to make further headway against the blaze, extending containment lines to 20 percent of the fire's perimeter, fire officials said.
The so-called Wallow Fire has destroyed or damaged three dozen homes and displaced up to 10,000 people while roaring through more than 700 square miles of tinder-dry ponderosa pine in and around the Apache-Sitgreaves National Forest of eastern Arizona.
The fire zone lies in the White Mountains, a picturesque area dotted with vacation cabins and popular as a weekend getaway for Arizonans seeking to escape the summer heat.
The conflagration erupted May 29, apparently from a campfire left unattended in a thickly wooded stretch of the Bear Wallow Wilderness area, just east of the San Carlos Apache Indian Reservation, said Forest Service spokesman Deryl Jevons.
Restrictions against open campfires in the forest had been lifted for the Memorial Day holiday weekend, following a fresh snowfall that forest managers believed had suppressed the wildfire threat, Jevons said. High winds and an abundance of dead, dried-out trees in the area combined to stoke the fire's early spread, he added.
More than 4,600 firefighters labored on Wednesday to consolidate their containment gains and further safeguard threatened towns on either side of Arizona's border with New Mexico before an expected return of high winds over the next few days.
"Today is a day to get some work done," Forest Service spokeswoman Suzanne Flory told Reuters. "We'll see what tomorrow brings, but for now we're looking pretty good."
Gains on the fire lines coincided on Wednesday with return of residents to the town of Nutrioso, one of several Arizona communities evacuated during the first week of the blaze. Two other towns, Alpine and Greer, remain off-limits to residents.
Evacuation orders were lifted on Sunday for 7,000 to 8,000 people forced to flee last week from another pair of towns near the border with New Mexico, Springerville and Eagar.
But authorities have warned returning families that lingering smoke and soot in the air pose health risks for children and people with respiratory problems.
The New Mexico town of Luna, an enclave of about 200 people less than 10 miles east of the Arizona border, was still on alert for possible evacuation but appeared to be out of immediate danger.
Controlled burns have been carried out on the outskirts of that town to remove brush and trees as potential fuel for advancing flames. But Forest Service officials say that so far the Wallow Fire itself has not crept into New Mexico.
The latest aerial infrared images of the fire on Wednesday showed that 478,452 acres -- or nearly 748 square miles -- have burned overall, surpassing the 468,638 acres charred in 2002 by the Rodeo-Chediski Fire in eastern Arizona. That makes the Wallow Fire the largest on record in Arizona.
The 2002 fire was far worse in terms of property losses, however, destroying 465 homes.
By comparison, the Wallow Fire has so far gutted 32 homes and damaged five others. Some three dozen nonresidential structures also have been lost. But no serious injuries have been reported.
(Writing and additional reporting by Steve Gorman; Editing by Greg McCune)