They also say the Lockheed P2V-7 that crashed shortly after take off Monday evening from Reno-Stead Airport had been inspected not long ago and was only about 36 hours through a normal 100-hour inspection schedule.
"What precipitated the fire, we don't know," said Tom Little, lead investigator for the National Transportation Safety Board.
"I asked the operator if they had ever experienced anything like this and they haven't," he said.
The plane owned by Neptune Aviation Services of Missoula, Mont., and built in 1962 was one of 12 the company had on contract with the Forest Service to fight fires.
Lockheed started building the planes for the U.S. military in the mid-1940s — "the predecessor of the P-3, the submarine chasers," Little said. "This was one of the later models."
Typically powered by propellers, the plane that crashed had been retrofitted with two additional jet engines for added thrust during take off, Little said.
It was one of the jet engines that at least two witnesses saw on fire shortly after take off. The flames engulfed the left wing before the plane went into a roll and crashed, killing all three members of the aerial firefighting crew, he said.
The Washoe County Coroner's Office identified the three victims on Wednesday as: Calvin Gene Wahlstrom, 61, Hunstville, Utah; Gregory Gonsioroski, 41, Baker, Mont.; and Zachary Jake Vander-Griend, 25, Missoula, Mont.
Gonsioroski's sister-in-law Kristi Freeman said the family was making funeral arrangements for Friday in Baker, a small eastern Montana town near the North Dakota border.
"He loved to fly, and I think he just enjoyed doing something that allowed him to help others," Freeman said.
Freeman said her sister, Kim, married Gonsioroski 13 years ago and the couple has three young children, ranging in age from 3 to 7. Freeman said she did not have any information about the crash, other than what has been reported in the news.
A cousin of Wahlstrom, Teri Busick of Huntsville, said the pilot's wife and brother were flown to the crash site Tuesday.
"It's a brutally dangerous job, probably the most dangerous job in our country," Busick told the Standard-Examiner of Ogden, Utah. "There are so few of them and so many deaths."
"He was a real good guy and the kind of person that everyone would want for a neighbor," Huntsville City Councilman Steve Johnson said of Wahlstrom.
Little planned another news briefing at the Reno-Stead Airport at 6 p.m. Wednesday. He told reporters on Tuesday that investigators had recovered several large pieces of metal beginning about one-quarter mile north of the runway that appear to have come from the burning engine.
"It appears it had disintegrated and subsequently left the aircraft," he said.
Little said they found nothing on the runway or surrounding area that would explain why the engine caught fire. He said he talked with officials for Neptune who helped survey the crash site on Tuesday, including one veteran pilot.
"He'd flown this kind of plane for almost 30 years and never seen this before. He said he'd never seen an anomaly of this type," Little said.
"There will be more research on our part to see if we find anything that might be associated with this, but right now we have an anomaly," he said.
"We have a fire. We have evidence that is factual in nature that the jet engine came apart. How that played in the loss of control, we don't know yet," he said.
"We have no communication from the pilot or co-pilot relative to an anomaly with the aircraft," he added.
Monday's crash marked at least the third time a P2V owned by Neptune suffered a fatal crash while fighting wildfires on government contract over the past 15 years. Two men were killed when one crashed near Missoula in 1994 and two other men died in a crash near Reserve, N.M., in 1998.
Neptune Aviation Chief Executive Officer Mark Timmons said those previous crashes were found to be caused by pilot error.
The Forest Service grounded 33 air tankers in May 2004 after an NTSB report said it was not known whether several types of air tankers were safe. The report was prompted by the crash of three C-130 air tankers.
A total of 27 crew members have been killed in crashes involving firefighting air tankers since 1991.
Neptune Aviation had several aircraft grounded by the decision, but it wasn't clear if the one that crashed Monday — Tanker 09 — was one of them.
A spokeswoman for Neptune in Missoula said Timmons was not available for comment on Wednesday. He had said on Tuesday the P2V had proven to be extremely reliable as an air tanker.
"I'm quite confident they are a safe platform," Timmons said.
Each airplane has undergone an inspection that takes at least a month to conduct, following fears in 1994 about using older planes, Timmons said. He said ongoing inspections, which include annual X-rays to look for cracks, is more intensive than those done on passenger planes.
"It is a dangerous business," he said. "We try to do as much as we can to decrease that amount of danger, but it is a dangerous business. There are risks in it."
Little said the air tankers are inspected by their owners every 100 flight hours. He said he didn't know the date of the crashed plane's last inspection but that it only had 36 hours of flight time on it since then.
"It had 64 hours more to go before the next inspection," he said.
"With the lengths these companies go to to ensure the plane's safety, an anomaly like this is just horrific. It is just a horrendous situation. It is catastrophic."