POST FALLS - The cause of the fire last week that destroyed a riverfront Post Falls mansion owned by a couple with financial woes will be undetermined, an investigator said on Friday.

Debris samples that petroleum-sniffing dogs hit on - the last pending part of the on-scene investigation of the former home of Len and Pam Wallace - came back from a lab as lamp oil, which doesn't point to arson, said Dan Ryan, Kootenai County Fire and Rescue division chief.

"We knew there was lamp oil there because they had lamps," Ryan said. "Lamp oil is a flammable, but not a high-rate-of-spread type of flammable. It's nothing that would point to where there was any wrongdoing.

"As far as what we've been able to do on scene, the cause will be undetermined."

The Wallaces' insurance company had security pulled from the scene on Friday.

Police, meanwhile, continue their part of the investigation.

The 11,000-square-foot home at 1504 E. Plaza, which was assessed at $1.5 million and built in 1986, has been owned by the Wallaces for the past 10 years. It was slated to be foreclosed upon the day after the fire, sparking suspicions of arson.

But Len Wallace on Friday didn't appear to be riled over the arson talk and said he wasn't necessarily relieved that fire investigators couldn't determine a cause.

"I don't know why I would be relieved," he said. "I think it would be better to know what the cause was."

In addition to the accelerant-sniffing dogs, KCFR brought in members of the State Fire Marshal's Office and the federal Alcohol, Tobacco, Firearms and Explosives to assist its own investigators.

Ryan said the extent of the damage - and structural problems as a result - made the investigation a challenge, but he believes a thorough check into the cause was made.

"The structure is so unstable that, to get in and do more hands-on investigating, would've taken an investment of some heavy equipment and week's worth of work to get through it," he said. "Since we didn't see anything that would lead us to a deliberate fire, the insurance company decided it wasn't practical go that route.

"Somewhere you have to decide on the risk vs. the gain. We went as far as we could with the investigation."

Part of the structure collapsed on Tuesday, confirming investigators' sense of danger, Ryan said.

"It's not a good place to be," he said.

Ryan said investigators are aware financial troubles for the Wallaces piled up - hence, assistance in the fire investigation was called upon - but he believes all practical avenues were exhausted in the search for a cause.

"There's lots of circumstantial evidence, but maybe that's all it is," he said. "Maybe a lot of bad things happened all at once for them."

Wallace said no one was home when the fire started, but he speculated the morning of the blaze that the cause may have been associated with one of seven fireplaces in the home. But he later realized it could've been electric-related.

"I thought (Pam) had left the upstairs fireplace on, but she's pretty sure it wasn't on," Len said. "My suspicion, looking back, is that it was electrical because there's so many wires that had been redone and redone and there's seven circuit breaker panels. There were lights everywhere in that house.

"But the answer is, 'We don't know.'"

Wallace said no one was home during the fire because the water had been shut off due to water damage caused by the dishwasher. The family is staying in a motel in the interim and plan to rent.

Wallace said he was hopeful he would be able to stave off foreclosing on the home through bankruptcy court.

"We were still in the mode of being able to save it," he said.

Police interviewed the Wallaces, along with their two male teens who lived in the home, and said all of their testimonies as to the circumstances before the fire were consistent.

Wallace said he was a victim of what he believes was a business fraud in Montana. He said he invested in a company that has products that identify dairy cows and, after he tried to get out and get his money back, there was a judgement against him.

The decision in 2003 awarded MagTrac Bolus, LLC, a $2.5 million judgement against Wallace for damages. Wallace then had five unsuccessful appeals to the Montana Supreme Court.

He had plans to develop about 900 acres of property south of Post Falls on Blossom Mountain known as Raspberry Ridge until the economy went in the tank.

Wallace operated the controversial Big Velvet Ranch in the 1990s south of Darby, Mont. It was an enclosed 2,000-acre elk farm used for "shoot for pay" hunting.