Mansion’s Explosion Uncorks a Mystery : Crime: Investigators sifting through the case of Napa Valley chateau find tangled motives involving tales of seduction and revenge. But the blast also destroyed much of the evidence.

All that is left of the Napa Valley’s most spectacular mansion is a big hole in the ground, a swimming pool and a whole lot of questions.

When the 18,000-square-foot wine country chateau exploded last April, it seemed to lift off like a rocket, then disintegrated into a heap of rubble. The heat was so intense, and the destruction so complete, that investigators were lucky to find some silverware.

Now, more than two months later, authorities are puzzling over a mystery that involves a $9-million insurance policy, tales of seduction, disputes over money, questions of revenge and a very clever bombing.

“Whoever did this knew exactly what they wanted to do and that was to completely destroy the residence with no traces,” said Lt. John Baumgartner of the Napa County Sheriff’s Department. “No one in the county has ever worked anything of this magnitude.”

Authorities are focusing their attention on the owner, an aggressive and successful developer named Joaquin de Monet, who was trying to sell the mansion when it was blown up. They also are looking at several of his business associates, his longtime friends and his enemies--which he appears to have in abundance.

Federal, state and Napa County agencies, as well as the Fireman’s Fund insurance company, are investigating the blast, but officials say it may be years before the case is solved.

The massive explosion just before dawn on April 27 woke residents for miles around and shocked this usually quiet, wine-growing and tourist region, home to 240 wineries and 110,000 people.

Completed just two years earlier, the opulent, two-story faux chateau with circular turrets and a commanding view of the vine-covered Napa Valley was something of a landmark.

“It was a beautiful house,” said George Van Noy, a retired soft drink distributor who lives in Napa. “I think it’s a shame someone would do something like that.”

The bombing itself was quite simple.

De Monet and his family were vacationing at Lake Tahoe and the house was empty. But fortunately for the bomber, or bombers, the sophisticated alarm system was turned off at the 40-acre hillside estate outside the city of Napa.

A propane tank used for heating the pool was near the mansion. The culprits connected a hose to the tank and began filling the mansion with the explosive gas.

The propane, heavier than air, spread through the lower floor then rose to the upper story. The gas filled the lavishly furnished living room, the huge foyer with the crystal chandelier, the six elegant bedrooms, the office suite, the fur vault, the music room equipped with the best in sound gear and the 10 marble bathrooms, including the one with a shower for the dog.

Within about 10 minutes, authorities speculate, the propane reached the attic and the mansion exploded, going straight up in the air and collapsing into a million fragments, virtually all of them smaller than a softball. Amazingly, most of the debris landed in what had been the cellar.

“They had ignition and liftoff,” said Baumgartner, who was awakened by the blast at his home three miles away.

Authorities have never found an ignition device, but speculate that a candle or flare was placed in the attic. Or that the propane was set off by the small electric charge of a ringing telephone. Or by a remote control device. Or by a tracer bullet fired into the house from a distance. No one is really sure.

Authorities say they are focusing on two possible motives for the bombing: money and revenge.

De Monet, a soft-spoken man of 48, had tried without success to sell the mansion. Initially, it was listed for $12.5 million, furnished. When there were no takers, he dropped the price to $8 million, unfurnished.

He purchased the property for about $1.5 million, spent about $4 million building the house and insured it for more than $9 million.

De Monet, who made his mark developing office buildings on the San Francisco Peninsula, had been stymied in recent months in plans to build a big office complex in East Palo Alto. There have been reports that he was in financial trouble and needed a quick infusion of cash.

But De Monet’s defenders deny that he needed money and pointed out that delays in receiving payment on the insurance would make it an impractical way to bail himself out.

De Monet has cooperated with investigators and his attorney, Robert Van Nest, says he is innocent. But the Napa County Sheriff’s Department has not eliminated him as a suspect and investigators are delving into his financial affairs.

One who believes in his innocence is Robert Vizcarra, De Monet’s longtime gardener, who was helping clean up several days after the explosion. “When he came here, he started to cry,” the gardener said. “Joaquin loved this house.”

De Monet declined to discuss the bombing. “We’d appreciate it if we didn’t talk about it. We want to hold off. We’re kind of trying to get our family back together.”

Over the years, De Monet has had disputes with various contractors and business partners, perhaps giving one of them a motive to blow up the house. Even those close to him concede he has a lot of enemies.

In the early 1980s, someone placed a pipe bomb in the roadside mailbox of his home in Woodside. The device destroyed the mailbox but did not injure anyone.

In recent months, De Monet has been engaged in a feud with one of his ex-wives. He charged in a civil suit filed in San Mateo County that she seduced his son from a previous marriage when the boy was 15. The affair, De Monet claimed, was kept secret from him and lasted more than five years.

The ex-wife responded, according to court documents, that her involvement with the youth was “merely extramarital conduct during marriage.”

After investigating the dispute, authorities concluded that she did not have sufficient motive to blow up the house and have cleared her as a suspect.

Investigators are paying more attention to the contractor who built the house and who has been wrangling with De Monet for payment of more than $400,000 he claims he is owed.

He knew the house well and was there 16 hours before the bombing. But the contractor’s attorney, Tom Nolan, said his client was home with his children at the time of the blast.

“I am absolutely 100% sure he didn’t do anything and didn’t know anything about it,” Nolan said. “It was something he cherished. It was a real piece of work.”

For the moment, the Sheriff’s Department, the U.S. Bureau of Alcohol, Tobacco and Firearms and the State Fire Marshal’s Office are attempting to learn what they can from the scant physical evidence. Investigators are trying to reconstruct the ashes found in a supposedly fireproof safe to see what the documents were.