DENVER — In 2009, a burly Colorado man named Rick Duncan was a rising star among local veterans groups, advocating on behalf of struggling soldiers and holding forth about his own powerful experiences returning from Iraq as a wounded Marine.

The problem was none of it was true, not even his name.

Mr. Duncan was actually Richard G. Strandlof, a troubled drifter who had never served in the military. Instead, he used his bogus story to work his way into the company of prominent politicians and admiring veterans.

Mr. Strandlof was eventually arrested by the F.B.I. and charged with violating the Stolen Valor Act, a 2006 law that makes it a federal crime to lie about being a military hero.

But though he admitted conjuring the entire tale, Mr. Strandlof has been fighting the case against him, arguing that the law violates his right to free speech. Simply telling a lie, his lawyers assert, does not always constitute a crime.