Calling himself "Don Campbell," a chunky, bearded man boarded a plane in Boston last Feb. 24 and flew to Seattle, Wash. The next day, posing as "Mr. Hutchinson," he cashed a stolen $16,000 check at a Seattle bank. That afternoon he used a stolen credit card to rent a Hertz car, signing as "William S. Adamczyk." Today, in Seattle's King County jail, he's lost his beard and is simply "John Doe."

Although he stood trial in April, when he was convicted of forgery and theft, and was sentenced to 10 years in prison June 21, Doe has never revealed his identity. Even after an international computer search of fingerprints and mug shots, he remains a mystery. Says Seattle detective Larry Baylor, "As far as the system is concerned, this man doesn't exist."

Doe maintains that he is 36, widowed, and the father of three children who now live in Pawtucket, R.I. But that information has been useless in helping police trace his identity, and authorities are skeptical of its accuracy. Some detectives speculate that Doe may be a government informant, or a federally protected witness. Seattle FBI spokesman Dave Hill does not give much credence to those theories, but can offer no other clue to Doe's identity. "He doesn't turn up in our national records. We don't know who he is." Alex Bortnick, the deputy prosecutor who tried the case, suspects that Doe may not be acting alone. He notes: "Doe knew a great deal about a lot of local people. Obviously, there's someone out there helping him."

Bolstering that suspicion, officials say that Doe had a mysterious visitor during his sentencing and that his longdistance phone calls from jail are paid for by an unknown friend. Doe admits forging the endorsement on the $16,000 check. But he claims he had permission from a secret source. "I can't go into it because it's a matter of my safety," he says. "The federal government knows all about me." The prosecutor remains unimpressed by such cryptic statements. Snaps Bortnick, "We think he's making it all up to disguise his real past."