Oct. 15 (Bloomberg) — William Koch held a senior executive of his Oxbow energy company captive for almost two days after discovering his concerns about a plan to avoid U.S. taxes on $200 million in profit, the now ex-employee said in a lawsuit.

Former Oxbow Senior Vice President Kirby Martensen alleged that an unrelated company probe revealed his misgivings over the purported effort by Oxbow Carbon & Minerals LLC. As a result, Martensen said, he was falsely imprisoned by Koch's agents and interrogated at a remote Colorado estate as part of an attempt to intimidate him. He was also fired, he said.

Koch, who Bloomberg Billionaires Index estimates is worth $3.7 billion, is the brother of conservative Tea Party funders David Koch and Charles Koch. Martensen, promoted last year to senior vice president, said he was lured to the billionaire's "Bear Ranch" near Aspen in March on false pretenses and questioned about anonymous allegations of wrongdoing made against him tied to a kickback scheme. The company sued him over the claims in Florida state court March 22, while he was allegedly being held against his will in Colorado.