"Thompson" is really John Donald Cody, who had eluded the FBI for 25 years, ABC News reported.
U.S. Marshal Peter Elliott said he discovered Thompson's real identity as Cody by matching up details about the two men from old FBI wanted posters. He said Thompson's unusual hairstyle, his history in Arizona and his knowledge of the law -- he's a Harvard law graduate -- were identical to those of Cody.
He noted that Thompson signed his name "Mr. X" and didn't want to be identified.
"Now we know why," Elliott said.
WJW-TV, Cleveland, reported the 65-year-old Hoboken, N.J., man had been a captain with the U.S. Army, working in military intelligence.
Cody, who used his fake identity to solicit funds for a fake veterans charity, was wanted for various frauds and questioning in an espionage investigation, officials said.
Thompson, or Cody, was arrested in Portland, Ore., in April for allegedly soliciting more than $100 million in donations to his charity, U.S. Navy Veterans Association, The (Cleveland) Plain Dealer reported. He was charged with identify theft, fraud and money laundering, and is being held in the Cuyahoga County Jail in Cleveland.