But in 2010, when Melgen first invested in GFS, the firm was actively considering marketing its natural gas engine devices for on-road vehicles. The bill, both in its 2009 and 2012 versions, authorized changes to IRS rules allowing larger tax credits for on-road, natural gas-supplied trucks and vehicles as well as grants for research. The proposal also urged the Environmental Protection Agency to streamline rules covering the conversion of diesel and gas engines to natural gas and alternative fuels.

GFS said in October 2010 in a press release that its strategy "integrates four related areas of business development," including "on-road coal truck conversions." The release, written by the company's consultant and then-director of strategic projects, Elio Muller, also said that "a vast number of on-road 18-wheeler tractor-trailer trucks hauling coal" in the Appalachian region of Kentucky and West Virginia could be converted to combination diesel-natural gas engines with the GFS system.

Muller, a former Commerce Department official in the Clinton administration involved in several Tampa businesses, said last week that he introduced Melgen to Green and other GFS officials in early 2010. Green also said Muller was instrumental in bringing the company to Melgen's attention. Muller said he has known Melgen from Florida's Democratic political circles dating back to the late 1990s. At one point, Muller drew up plans to start a business, Melgen & Muller Inc., but the men never followed through.

Melgen has made investments in health-related companies since the 1990s, according to SEC reports, but his GFS stake is his only evident natural gas-related investment. Green said he met several times with Melgen and found him to be an "intelligent investor" but could not explain his sudden interest in natural gas.

"I don't know how he found out about natural gas, but he liked what we were doing and thought it was innovative," Green said.

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