A former executive of Oxbow Carbon has accused company founder and chief executive William “Bill” Koch of kidnapping and holding him captive at Koch’s Bear Ranch in Gunnison County.

Kirby Martensen, formerly senior vice president-Asia with Oxbow Carbon and Minerals International, sued Koch and others in U.S. District Court for the Northern District of California on Thursday.

Martensen said Koch invited him in March to the 420-acre ranch, where the billionaire has built a faux Old West town with 50 buildings. While there, Martensen was questioned about allegations that he and other employees had engaged in theft, fraud and self-dealing, he claims.

He was taken for a drive and served termination papers and a lawsuit, Martensen said. He was placed in a cabin and “imprisoned” and held in “captivity” for three hours.

Martensen asked to be driven to Aspen for his scheduled flight back to California but was instead “kidnapped and kept captive,” while being driven to a small airport near Denver, he alleges.

At about 2 a.m. on March 23, Martensen was ordered into a private plane and was flown to Oakland, where he arrived at 4 a.m., he said.

Martensen alleges he and his family’s relocation to Oxbow’s Singapore office “was part of a plan being implemented to evade paying taxes to the United States on profits in excess of $200 million a year.”

He claims that Koch learned that he and others were concerned about the legality of what they were doing on behalf of Oxbow and their “distrust of upper management.”

“As a result, William Koch promoted and implemented a plan to intimidate and discredit (Martensen) for the purpose of chilling his speech and credibility,” Martensen alleged.

Oxbow spokesman Brad Goldstein

denied the allegations Monday. The West Palm Beach, Fla., company sued Martensen and two other Oxbow employees in March, accusing them of participating in a wide-ranging scheme to misappropriate revenues. The suit accuses the former employees of accepting bribes, kickbacks and payments from competitors.

Goldstein said that for 16 years, Martensen was a highly compensated employee of the company — “a senior vice president which made his betrayal even more distressing.”

Regarding Martensen’s allegations about taxes, Goldstein said that “the fact we have overseas tax issues is commonplace for an international company.”

“We abide by all domestic and international laws,” he said.

In a statement, Oxbow said several of Martensen’s co-conspirators are now cooperating and have “admitted culpability.”

“Any allegations of misconduct by Mr. Koch simply are untrue and stem from Martensen’s attempts to divert attention from his own wrongdoing,” the statement added.

Martensen is asking for compensatory damages exceeding $75,000 and punitive or exemplary damages of more than $75,000.

Forbes estimates Koch’s net worth at close to $4 billion.

Howard Pankratz: 303-954-1939, hpankratz@denverpost.com or twitter.com/howardpankratz