(Adds crash details; background on previous fire deaths)
By Jennifer Dobner and Zelie Pollon
June 4 (Reuters) - Officials opened an investigation on Monday of an airplane tanker crash in Utah that caused the year's first two deaths among crews fighting U.S. wildfires, while hundreds of evacuees from a New Mexico blaze that is the nation's biggest this season returned home.
The firefighting plane went down on Sunday afternoon on a forested mountainside in the Hamlin Valley area of southwestern Utah while on a mission to drop chemical fire retardant on an 8,000-acre (3,237-hectare) blaze along the Nevada-Utah border.
Firefighters who accompanied a search team to the crash site hours later battled flames roaring 100 feet (30 metres) in the air as sheriff's deputies extricated the bodies from the wreckage and collected crash evidence, said Detective Sergeant Jody Edwards of the Iron County Sheriff's Office.
The charred ground at site was so hot that "the soles of our investigators' boots were melting, and there were places where the aluminum from the plane had literally turned to liquid and was running downhill," Edwards told Reuters.
Debris was scattered across the mountainside, with large pieces of wreckage about 500 yards (457 metres) from the last spot where the plane appears to have dropped retardant, he said.
A National Transportation Safety Board team was called in to investigate the crash.
Based on damage to the trees and marks left in the ground, sheriff's deputies on the scene surmised that the plane may have clipped some trees with its left wing on a low pass toward the mountain, sending the aircraft into a cartwheel, Edwards said.
The dead were identified as pilot Todd Tompkins, 48, and his co-pilot, Ronnie Chambless, 40, both of Boise, Idaho. The two men were employees of a Montana-based aviation company that supplied the plane, a Lockheed Martin P2V, and crew under contract with the U.S. Forest Service for firefighting.
Don Smurthwaite, a spokesman for National Interagency Fire Center in Boise, said Tompkins and Chambless were the first firefighting personnel to die in fires that have consumed more than 1,200 square miles (3,108 square kilometres) of forest, brush and grasslands nationwide so far this year, most of it in western states.
But they were not the only wildfire casualties of 2012. Three civilians - an elderly couple and another woman - died in a blaze triggered by embers from a prescribed burn west of Denver in late March.
Still, the year-to-date tally of burned acreage across the country is running about 40 percent below average for the same six-month period over the past decade, said Ken Frederick, a spokesman for the National Interagency Fire Center in Boise.
As of Monday, firefighters were battling a total of 11 large, uncontained blazes, mostly in seven Western states - New Mexico, Arizona, California, Nevada, Utah, Colorado and Idaho.
The biggest by far is the so-called Whitewater-Baldy Complex fire, which was ignited by lightning on May 16 in the rugged high country of New Mexico's Gila National Forest and has now scorched more territory than any other recorded blaze in the state's history, over 255,000 acres (103,195 hectares).
The blaze destroyed a dozen privately owned cabins at the height of its rampage nearly two weeks ago as gale-force winds fanned flames from treetop to treetop.
But calmer winds in recent days have allowed fire crews to gain an upper hand. By Monday, they had carved containment lines around 18 percent of the fire's perimeter and were depriving advancing flames of fresh fuel by clearing smaller trees and brush that has yet to burn.
"The wind has really been in our favor the past couple of days," Fire Information Officer Tara Ross said. "It's still warm and it's still dry, but with low winds we can ... lower the intensity of the burn."
Evacuation orders were lifted on Monday for about 200 homes in and around the historic mining town of Mogollon, but about 60 cabins in the nearby community of Willow Creek, where 12 cabins and 13 outbuildings were lost on May 23, remained off-limits.
Ross said containment work was concentrated along the fire's western flank to protect the community of Glenwood and its population of about 300 people. Teams on horseback scouted areas along the eastern edge of the blaze, looking for ideal places to create new fire breaks there, she said.
Lightning strikes remained a problem, however, blamed for sparking three new fires in New Mexico on Sunday. Lightning was also blamed for the White Rock Fire that erupted on Friday in southeastern Nevada near the town of Caliente and later burned into Utah. It was that fire which claimed the lives of the two air tanker pilots. (Reporting by Jennifer Dobner from Salt Lake City and Zelie Pollon from Santa Fe; Writing by Steve Gorman; Editing by Cynthia Johnston and Vicki Allen)