John Doe walked into some dense woods in Prince William County in early May, put a gun in his mouth and pulled the trigger, according to police.

Beyond that, there is little they know about the dead man: not who he is, where he came from or why he did it. He left no note and no identification.

His apparent suicide has prompted an extensive identification effort in Prince William County, one that will eventually cost county taxpayers thousands of dollars. It already has involved extraordinary amounts of police time for a suicide since the man's body was found by hikers on May 12.

Police have brought in the FBI and the Bureau of Alcohol, Tobacco and Firearms, as well as scientists from the Smithsonian Institution and the state Department of Agriculture.

Authorities have analyzed the .38-caliber Titan handgun found in the man's lap, run his fingerprints through a national criminal record computer, examined missing persons reports and dental records, and questioned dozens of people.

Still there is no clue to his identity, but police continue to try to piece together the unsolved mystery.

"A lot of people probably think that if we can't ID a body we just let go, but we can't close this case until we know who he is," said Prince William County police Investigator Ron McClelland, stroking the three-inch-thick folder he has compiled on John Doe.

"This case could remain open for years if we aren't able to find out who he is."

Police say Prince William's investigation is standard procedure in other jurisdictions in trying to identifyanonymous suicide or homicide victims.

This John Doe makes a good example of the unusual effort police must take when a victim dies alone with no identification.

John Doe is the third person listed as an unidentified dead person in Prince William County police records, said police spokeswoman Kim Chinn.

The other John Doe is an Asian man who burned to death in a car on Interstate 95 about two years ago, and a Jane Doe whose skeleton was found on Bull Run Mountain on Feb. 11, 1982.

Most of the people who kill themselves leave a note. When investigating a suicide scene, police often find neatly stacked insurance policies, bank statements and religious articles accompanied by explanations of the person's final desperate deed or instructions for family members, police said.

But John Doe, whose body was found May 12 in woods near Route 1 and Highway 123, left nothing.

He had no wallet or written identification or unique jewelry, and police are convinced the body was not robbed.

His body, which lay rotting against a tree trunk undisturbed by anything except beetles and maggots, had no tattoos or birthmarks. The body's only mark was a scar on the right arm.

"It's frustrating because you know he's got a family out there someplace," McClelland said.

Police have no idea whether the man intended to be anonymous in death.

What they do know is this: John Doe was a white male, 5 feet 10 inches tall, and weighed 197 pounds.

He had brown hair and was balding. He was dressed in a white button-down shirt, size large, with thin blue vertical stripes, over an orange shirt.

He wore size 36 blue work pants, white socks and white size 9 1/2 Reebok tennis shoes.

Lying on his chest was a pair of bifocal glasses with purple plastic frames, manufactured by Zimco.

An autopsy revealed his blood type was A, and he was circumcised.

McClelland believes the man might have been a vagrant who ended it all because he was alone and unhappy, and the investigator contacted Washington area homeless shelters asking if they can identify the man.

The Reeboks contradict the vagrant theory, however, as few poor people can afford such expensive shoes, he said.

The date of death was set at May 1 to 5, largely based on a study of maggots found on the body.

Crime scene technicians made imprints of John Doe's fingerprints in a clay-like substance, because the body had decomposed to a point where the usual ink-stain fingerprints could not be made, McClelland said.

The prints were then checked, some of them manually, against sets on record with the FBI, which came up with no match.

The FBI also gave McClelland a list of about 200 names of missing persons from around the country with John Doe's general description, but so far none has matched John Doe.

Police departments from around the country flooded Prince William with dental records of their missing persons, but again they yielded no match.

Police decided they needed the public's help. A photo was out of the question, because much of John Doe's face had been eaten away by insects. But a Smithsonian Institution anthropologist and FBI artist were able to fashion a drawing after reconstructing John Doe's face based on his general facial characteristics and bone structure, said Robert Mann, a Smithsonian physical anthropologist.

Police are scheduled later this month to recanvass the neighborhood where the corpse was found, showing the drawing to store employees, postal officials and Neighborhood Watch volunteers, McClelland said.

McClelland has compiled a list of optometrists and eyeglass manufacturers, who are being interviewed about the purple glasses.

The Bureau of Alcohol, Tobacco and Firearms traced the rare .38-caliber Titan gun to Hartsell, Ala., but discovered the store that sold it is now out of business.

The gun has not been reported missing, McClelland said.

Police also are looking to see if any cars towed in the county during that time have not been collected.

"It's likely that he might have driven to a spot near there and the car was towed away," McClelland said.

The owners of uncollected cars will be contacted to see if they have any information on the dead man.

"It's one thing we can do," he said.