DENVER (AP) -- Air safety investigators are examining whether age was a factor in the deadly crash of a World War II-era air tanker, the second plane in little over a month to break apart while battling wildfires in the West.
The nation's firefighting fleet of 32 heavy air tankers was grounded Friday as a precaution.
"We haven't ruled out anything yet," said David Bowling, a National Transportation Safety Board safety specialist.
The PB4Y-2 Privateer broke apart Thursday while battling a 2,500-acre wildfire 45 miles northwest of Denver. Both crew members were killed. Their names were not released.
Bowling said investigators would look at the age of the plane and the maneuvers required during low-level runs to drop its 2,000-gallon load of fire-retardant slurry on wildfires.
He said engine parts were found in several locations, and an explosion had not been ruled out.
Four other PBY4-2 tankers will not fly until federal investigators figure out what went wrong.
On June 17, a C-130A air tanker crashed after its wings snapped off near Walker, Calif. That prompted officials to ground the five remaining C-130A firefighting aircraft, which have not yet returned to the air. The accident is still under investigation.
The grounded planes are 42 years old on average. Industry experts said they believe the aging fleet could continue flying safely.
"Older airplanes do not mean unsafe airplanes," said William Broadwell, executive director of the Aerial Firefighting Industry Association. "The military flies airplanes that are exactly as old. As long as it's maintained properly it's a safe aircraft."
Both tankers that crashed were owned by Hawkins & Powers Aviation Inc. of Greybull, Wyo. The tankers were under contract to the Forest Service.
Ryan Powers, an assistant for Hawkins & Powers, said the company's planes are inspected by both the Federal Aviation Administration and the Forest Service.
"You can't simply say it's an old airplane. A lot of newer aircraft have a lot more time on them," he said.
The tankers' absence was felt at the wildfire where the Privateer crashed. Flames burst through containment lines and doubled in size Friday. Smoke and ash drifted into Estes Park, a tourist town at the busy east entrances to Rocky Mountain National Park.
Ten of the grounded tankers had been fighting fires in Oregon, which has more fires burning than any other state, officials said.
In Washington, wind-whipped fires threatened dozens of residences Friday night, prompting the evacuation of nearly 300 homes.
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