SALT LAKE CITY (AP) _ A ''shadow person,'' possibly a computer whiz or professor, has planted 12 bombs in six states over nine years and may be someone with a grudge against universities, professors, airlines or computer businesses, authorities say.

The bombs have killed one person and injured 21. The search for the person who painstakingly built them is centered on Utah, and rewards for his capture and conviction total $60,000.

The bombings began on May 26, 1978, at Northwestern University in Illinois. One was planted in an airliner en route from Chicago to Washington, D.C., and 12 passengers were treated for smoke inhalation. The most recent occurred just over a week ago outside a Salt Lake City computer store.

''I don't like him,'' said Salt Lake City Police Capt. Brent Davis, a member of the task force tracking the bomber. ''He doesn't care who he blows up, obviously.''

A slender, blond man dressed in a hooded sweatshirt, jeans and sunglasses ambled into the parking lot of CAAMS Inc. on Feb. 20 and, in full view of two employees, put a pipe bomb encased in wood and metal next to a parked car.

''This guy's getting a lot of confidence,'' said FBI agent Lou Bertram. ''In fact, he's so cool and calm that when the two witnesses saw him set the package down they said, 'Oh well, he's not trying to hide anything, we'll investigate it later.'''

But before they did, another CAAMS employee returned from a service call and picked up the package. It exploded and he was wounded in the face, arms and legs.

It was the first time the bomber had been seen by witnesses. Their descriptions produced a composite sketch distributed last week to law enforcement agencies nationwide.

The FBI has made public a few snippets of its psychological profile of the bomber, described by Davis as ''a shadow person.''

''It is apparent that from the extraordinary amount of time the bomber consumes in designing, constructing and assembling these bombs that he derives significant psychological fulfillment,'' according to the profile. ''His secretive nature indicates he is an extremely inadequate individual who fears any open confrontation with his real or imagined enemies.''

''Just based on his track record,'' said Davis, ''we would have to conclude we're dealing with somebody who's obviously quite bright. In a period over eight years, it doesn't appear that he's blown himself up yet.''

Eight of the bombings occurred in California, Illinois and Utah, the others in Michigan, Tennessee and Washington state.

Ron Wolters, an agent of the federal Bureau of Alcohol, Tobacco and Firearms, said the Feb. 20 bombing was tied to the others by analysis of the bomb's components, which included nails, screws, gunpowder and pipe.

FBI agent Robert Bryant said there was ''an absolute link.''

The bombs have been assembled out of pipes, gunpowder, gasoline, nails, screws, scrap metal, fishing line and rubber bands, among other things. Their containers have been elaborate hand-made wood boxes, metal cans, a plastic check file box, a cigar box, a book or paper wrappings.

''The bomber has spent a considerable amount of time building electrical components and switching mechanisms when low-cost, over-the-counter electric components are readily available,'' the task force noted.

Investigators are concentrating their efforts in Utah because bombs that exploded at a University of Michigan professor's home and at Vanderbilt University in Nashville were mailed from Salt Lake City and Provo.

''It would lead you to believe there's more than a passing interest here,'' Davis said. The FBI thinks the serial bomber may still be in Salt Lake City.

Six bombs that were mailed were addressed to specific individuals and two were preceded by letters announcing the arrival of a package.

''The bomber apparently has some college-level education as suggested by these letters, and he may be quite intelligent,'' the task force evaluation said.

The U.S. Postal Inspection Service increased its reward Wednesday from $25,000 to $50,000 for information leading to the arrest and conviction of the bomber. The University of California at Berkeley, where two bombs have exploded, has put up $10,000.

When the composite drawing was released Monday, authorities also set up a hotline that has been ringing incessantly.

Says the FBI's Bertram, ''Every call is a lead.''