TDPK was busted by the FBI and U. S. Army CID for impersonating a Department of Defense police officer when he tried to have some documents copied at an Indianapolis print shop. It was that arrest that led to a search of the car that he was driving. That search turned up the bomb making materials that led to his being charged as the Speedway Bomber.
Mark Singer adds this on page 94 of his book Citizen K:
Halloween was still a few weeks away, but Brett was already in costume. He had on navy-blue trousers, a medium-blue short-sleeved shirt with a sew-on cloth Department of Defense Police shoulder patch, and a gray wide-brimmed felt hat. The overall effect—especially the Smokey the Bear flourish of the hat—bordered on slapstick. The eventual charges against Kimberlin were impersonating a Department of Defense officer, illegal possession of military insignia, and illegal possession of a facsimile of the Great Seal of the President of the United States. The latter two offenses were so obscure that Kimberlin said his later search of case law turned up no other criminal prosecutions under the relevant statutes—a statistic that seems to fit the novel circumstances under which he got himself busted.
Others have been convicted of terrorist crimes such as bombing, others have been convicted of impersonating government officers, but to be the first, and only, one convicted of illegal possession of the Presidential seal … Now, that’s a unique place in the annals of crime!
UPDATE—Mark Singer notes the “slapstick” appearance of TDPK in his getup. I note that his beard is a sufficiently gross violation of the grooming standards in place at the time that no DoD personnel, military or civilian, would have believed he was for real.