POST FALLS - The fire that destroyed a gated Post Falls mansion on the Spokane River next to the "Amway house" Thursday morning has investigators digging for answers.

"We're not ruling out anything," said Kootenai County Fire and Rescue Chief Ron Sampert.

No one was inside the 11,000-square-foot, $1.54 million home on the north side of the river at 1504 E. Plaza, said Len Wallace, who owns the home with his wife Pam.

Wallace speculates the fire may have been caused by one of seven natural gas fireplaces in the home.

"There were heating problems in the house," he said.

But investigators said a cause hasn't been determined as the charred two-story home built in 1986 remained too unstable to search for a cause on Thursday afternoon.

"I looked out the window and saw flames 30 to 40 feet in the air," said Louise Fields, who lives three blocks away. "The whole house was engulfed in flames. It was the worst black smoke I've ever seen."

Wilamena Berg, a neighbor who lives on property adjacent to the home, said she was awakened by loud "popping" sounds outside her house.

She realized they were explosions after going outside.

"I was concerned the wind might blow the flames over to my house," Berg said.

Post Falls police and Kootenai County Fire and Rescue are working together on the investigation.

"We'll look at it as a criminal investigation until we're able to prove otherwise," Police Chief Scot Haug said. "There's a lot of unknowns that we need to find out about what occurred."

Wallace, a developer who owns property south of Post Falls on Blossom Mountain known as Raspberry Ridge, said he has owned the home for 10 years and it's in the foreclosure process.

"I figured that I could save it (from foreclosure)," Wallace said.

Neighbors reported seeing items removed from the home in the days before the fire.

"We have been moving some things out because we're in an unstable situation," Wallace said.

Wallace said he's also going through bankruptcy proceedings because he was a victim of what he believes was a business fraud in Montana. He alleges the fraud occurred when he invested in a company that manufactured products that identify dairy cows. He said when he tried to get out and get his money back, a judgment was filed against him.

Wallace ended up owing the company he invested in, Magnus Bolus, LLC, $2.5 million in damages as the result of arbitration. Wallace then made five unsuccessful appeals to the Montana Supreme Court. He is still pursuing the matter in court.

Wallace said that due to water damage caused by a dishwasher in the house, he and other family members were not staying there when the fire was reported by neighbors at 6:20 a.m. Wallace said he was staying at a large shop on property he owns south of the river, while Pam was at a friend's house and two other family members in their late teens were staying at a hotel.

When firefighters arrived, they battled the blaze in a defensive mode because the fire had already engulfed the home.

"It went up in a hurry," Post Falls Police Capt. Greg McLean said.

Wallace said that, due to the water damage from the dishwasher, the water had been shut off. He said two of the fireplaces heated the home.

Wallace said the home had seven bedrooms, an unfinished indoor swimming pool, 47 skylights and an elaborate interior with cherry woodwork, tile and mirrors.

"It was a beautiful home," he said. "I can't believe with all the brickwork and Sheetrock that it would burn like that."

Wallace said it is believed a ferret died in the fire.

"We didn't have valuable paintings and jewelry, but there's mementos in there," he said, adding that his books and records are at his office at his shop.

Wallace said the garage was full of "stuff," including holiday decorations. He didn't immediately know if there were any vehicles in the garage.

A guest house formerly owned by the Wallaces, and situated on the east side of the destroyed home, was not damaged by the fire, nor was the 26,000-square-foot Amway house to the west of the Wallace's home.

The Amway house is owned by Ron and Georgia Lee Puryear, who joined Amway 40 years ago.

Police reports indicate officers responded to the Wallaces' home on Wednesday night after a 911 hang-up call. Pam Wallace told police that she and Len had a verbal argument over a set of keys. Len had left the home by the time officers arrived.

No citations were issued.

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.

Staff writer Maureen Dolan contributed to this report.

An underground fire was reported Tuesday night just before 8 p.m. at the Sunshine Mine near Kellogg. As of 1 p.m. Thursday, the fire was not extinguished.

According to a preliminary accident report from the U.S. Mine Safety and Health Administration, there were 12 contracted Redpath employees underground at the time of the fire's discovery. No one was injured, and all are accounted for.

Workers were notified of the fire, the report states, because safety alarms were activated.

"The mine was safely evacuated, with all miners accounted for," the report states. "Subsequent CO readings indicated a fire underground."

The cause or location of the fire has not been released. Calls to Operations Manager Robert Higdem were not returned Thursday.

The mine is owned by Silver Opportunity Partners. It currently employs 68, with 22 underground workers and 46 working in other areas.