Billionaire William Koch is facing a lawsuit in federal court from a former top-level employee who claims the energy mogul lured him to a secluded property, where he was imprisoned and interrogated for a period of time, according to a report in Courthouse News.

John Houston Scott, an attorney for Kirby Martenson, a former executive for a number of Koch subsidiaries, confirmed the story to HuffPost, adding that he guessed the case could go to trial in a year.

The case was filed in San Francisco, where the Republican-backing Kochs are not deeply popular.

Martensen alleges that the multiple-day-long incident was set off by an anonymous letter accusing Martensen of mismanaging and stealing from Oxbow Carbon & Minerals International while serving as the company's vice president. That accusation prompted the company's leadership to read Martensen's letters and emails without his knowledge, the suit charges, which led the company to discover that Martensen had deep reservations about the legality of the company's tax avoidance strategies.

Martensen alleges he was told by his superiors that he was transferred overseas to Asia so that the Koch company could avoid paying U.S. taxes on some $200,000,000 in profits.

“Kirby Martensen states in a lawsuit that we investigated him for participating in a wide-ranging scheme to defraud, accepting bribes and diverting business from our company," Brad Goldstein, director of corporate affairs for Oxbow, a Bill Koch company, said in a statement. "He is right. We absolutely investigated Martensen and determined that he did participate in the fraud against the company. We identified who was defrauding us and are pursuing appropriate action to hold them accountable. In fact, several of the wrongdoers have admitted their involvement and one has directly implicated Mr. Martensen in the scheme. ”

Koch, who is David Koch's twin brother, is estimated to be worth $4 billion. He previously made news by reconstructing a replica of an Old West ghost town on the isolated ranch where Martensen and his co-workers were allegedly held.

Martensen had a flight booked from Aspen, but was not permitted to travel to the airport, he claims, as a local sheriff worked with Koch's employees to "make sure you don't wander off," according to the complaint. He says he was subjected to a several-hour interrogation, fired, held against his will on the property, then forced to fly in a private plane to San Francisco, accompanied by a man he believed to be armed.

Through his privately held companies -- Oxbow Carbon and Huron Carbon -- Bill Koch has given $3 million to the pro-Mitt Romney super PAC Restore Our Future. His more well-known brothers have given substantially more to the effort to elect the Republican presidential candidate.

"Any allegations of misconduct by Mr. Koch simply are untrue and stem from Martensen’s attempts to divert attention from his own wrongdoing," said Goldstein.

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