This is absolutely sick.  It may be in Illinois, but don't think they wouldn't do this to someone in Wisconsin either:

David Wetzel, 79, was surprised to hear a knock on the door at their eastside home while he was still getting dressed.

Two men in suits were standing on his porch.

"They showed me their badges and said they were from the Illinois Department of Revenue," Wetzel said. "I said, 'Come in.' Maybe I shouldn't have."

Gary May introduced himself as a special agent. The other man, John Egan, was introduced as his colleague. May gave the Wetzels his card, stating that he is the senior agent in the bureau of criminal investigations.
...
The agents informed the Wetzels that they were interested in their car, a 1986 Volkswagen Golf, that David Wetzel converted to run primarily from vegetable oil but also partly on diesel.

Wetzel uses recycled vegetable oil, which he picks up weekly from an organization that uses it for frying food at its dining facility.

"They told me I am required to have a license and am obligated to pay a motor fuel tax," David Wetzel recalled. "Mr. May also told me the tax would be retroactive."

Since the initial visit by the agents on Jan. 4, the Wetzels have been involved in a struggle with the Illinois Department of Revenue. The couple, who live on a fixed budget, have been asked to post a $2,500 bond and threatened with felony charges.

Apparently Illinois isn't interested in alternative fuel plans.  Via The Agitator.