They have thought he might be a sullen student or teacher. They have thought he might be an enraged airline or computer worker. They have thought he might be someone at the post office or in a lumberyard. They have thought he might work at a factory that makes artificial limbs.

When he resurfaced last Saturday, mailing a package that killed an advertising executive named Thomas J. Mosser at his home in North Caldwell, N.J., investigators once again found themselves grimly sifting for clues to a bomber who has baffled them for 16 years and now seems to have escalated his rampage. The early returns, investigators said, have been achingly familiar -- no especially tantalizing leads to freshen a scent that remains faint at best -- but the searchers hint that they are exploring some new directions.

A $1 million reward, posted in 1993, still awaits anyone who helps find the wiliest mail bomber in modern history, one who has struck 15 times, killing 2 people and wounding 22. In further hopes of eliciting leads from the public, investigators have deliberately chosen to share an unusual amount of information in the case, which they call Unabom because of the early university and airline targets.

Extensive interviews last week with investigators, including officials of the Federal task force based in San Francisco assigned to the case, reveal a more detailed picture of a remarkably exhaustive manhunt -- thousands of leads, hundreds of suspects and endless computer searches into everything from the initials of someone's name to the phases of the moon. At one point last year, the trail led to an ex-convict who disappeared after he was questioned.

And yet, to the consternation of investigators, all the thousands of clues and theories have produced no solution. They do not know who the bomber is.

But some intriguing links have emerged. One of the more captivating ones is that the bomber is trying to say something through wood or trees. First, authorities noticed that all the bombs had various forms of wood in them, an illogical material rare in an explosive. The second device actually had tree twigs glued to it.

Then there are the names. One target was Percy Wood, who lived in Lake Forest, Ill. He got a bomb inside a novel, "Ice Brothers," published by Arbor House, whose symbol is a tree leaf. Moreover, some of the fake return addresses the bomber wrote on parcels referred to trees or wood. One listed Ravenswood, another Forest Glen Road. It may mean nothing, or it may mean a great deal, that Mr. Mosser lived on Aspen Drive.

To investigators who have spent years staring at this maddening puzzle, the recurring wood motif suggested its own swarm of possibilities. Was the bomber a crazed environmentalist? A demented lumberjack? Or, in an enigmatic case flush with blind alleys and an interminable web of theories, was it one more ploy in a macabre game?

The Federal Bureau of Investigation, the Postal Service and the Bureau of Alcohol, Tobacco and Firearms are all actively searching for the bomber. Jim R. Freeman, the special agent in charge of the San Francisco office of the F.B.I., is flying to Washington to personally brief Louis J. Freeh, the F.B.I. Director, on Monday. How close are they to catching him?

"How close?" asked Mr. Freeman. "One phone call away with the right tip. One computer run. Or years away." A Pattern Emerges Computers, Planes And Lots of Wood

It was the third bomb, in November 1979, that first outlined the dimensions of the problem.

An explosive designed to go off at high altitude caught fire without exploding in a mailbag aboard an American Airlines flight from Chicago to Washington. Twelve passengers suffered smoke inhalation. The bomb, nearly intact in its juice-can container, was dissected by an F.B.I. bomb examiner in Washington named Christopher Ronay. The label was scorched but seemed to be addressed to a business ending in "lines" in northwest Washington. Since the bomb was mailed in Chicago, Mr. Ronay sent photos to the Chicago authorities.

The photos reminded the Chicago police of a device found in May 1979 at the Tech building of Northwestern University in Evanston, Ill. They sent the remains to Mr. Ronay.

"Both had two initiators, which is unusual, that were identical," he said. "Each was carved in a round dowel form with grooves cut into it on each side for the wire. There were identical wire leads, and so on. I had never seen the design before in the thousands of bombs I had examined. The chances of two different people making them were nil. We knew we had a serial bomber."

Some time later, the F.B.I. learned of an earlier bomb found in an unmailed package in May 1978 in the engineering department's parking lot at the Chicago campus of the University of Illinois. Mr. Ronay concluded that it, too, came from the same hands. Presumably, it was the bomber's debut.

For tactical reasons, the authorities did not yet announce the presence of a serial bomber. And, given the evidence they had, they treated it as a Chicago matter.