However, replacing the aging fleet will not happen quickly, Harbour added. A contract for three air tankers will be awarded later this month, and four more will be added next year, he said.
The Forest Service also hires hundreds of helicopters, which drop water on hotspots within a fire, and dozens of smaller single-engine crop-dusters converted to carry retardant, which are largely used on range fires.
The service doesn't own any of the firefighting aircraft, but instead contracts with private companies like Minden and Neptune, whose fleet is made up largely of the P2Vs.
A review of firefighting plane crashes over the last two decades found that various models of the P2V aircraft had been involved in at least seven fatal crashes while fighting wildfires, including the one that crashed on Sunday.
Neptune Aviation, which owns five of those planes, reported several hydraulics failures on their P2Vs, one that led to an emergency landing in Montana in April. Another one was taken out of service in February after workers found a crack in the wing support.
Neptune released a statement Monday afternoon that said it would not comment on accident specifics, but noted the aircraft "made contact with the ground while flying in the active fire drop zone."
The company temporarily grounded its fleet to debrief crewmembers and mechanics, but all of the company's air tankers have been returned to active duty, the statement said.
National Transportation Safety Board investigator Van McKenny arrived in the area Sunday night and spent the day Monday interviewing witnesses, but he hadn't yet been to the crash site.
He said a team of investigators was assisting in the probe, including from the Federal Aviation Administration and the Interior Department.
Authorities will be looking into all potential causes of the crash, including weather, mechanical failure and pilot error.
McKenny said he planned to visit the crash site Tuesday to document the wreckage.
"It will all really depend on what we see, evidence on the ground," he said.
Authorities identified the pilots as Todd Neal Tompkins and Ronnie Edwin Chambless, both of Boise, Idaho.
Tompkins' wife Cassandra Cannon said her husband had flown air tankers for 17 years and believed the work he did was meaningful and impacted the safety of others. She said Tompkins was dispatched to the wildfire Sunday and immediately began flyovers.
Associated Press writer Matt Volz in Helena, Mont., Jeff Barnard in Grants Pass, Ore., and Todd Dvorak in Boise, Idaho, contributed to this report. Sonner reported from Reno, Nev., and Skoloff from Salt Lake City.