After uncovering a breakthrough clue, the FBI has offered a $1 million reward in the hopes of convicting a serial mail bomber who has targeted university professors for the past 15 years.

The clue is a personal message--"Call Nathan R. wed 7 p.m."--discovered on material mailed by the bomber in June to his last target, Yale professor David Gelernter, said FBI investigator Rick Smith., a member of the FBI's Unabomber Task Force in San Francisco.

Investigators believe that the "Unabomber"--so named because most of the incidents have occurred at universities--may have called Nathan R. the evening of the June bombing.

FBI Director Louis J. Freeh is appealing to thepublic and to Nathan R. to call the toll-freenumber, 1-800-701-BOMB, and provide anyinformation that might be linked to the bomber.The reward is offered for evidence leading to aconviction.

The phone number and release of the clue arethe FBI's latest attempts to nab the serial bomberwho has eluded investigators since 1978. Thebomber has sent 12 mail packages which have killedone person and injured 21 others, according to theFBI.

The most recent attack left Gelernter deaf inone ear, blind in one eye, missing a part of hisright hand and wounded in the chest.

Following the Gelernter bombing, which occurredthis past June, Harvard professors and studentsreceived precautionary information from HarvardPolice authorities on how to recognize possiblemail bombs and who to contact if necessary.

While bomb squads were called to the ScienceCenter and the house of Dean of the Faculty JeremyR. Knowles to investigate suspicious packages, nobombs have been discovered at Harvard since theJune bombing at Yale.

Many of the bombings have occurred in November,and the Unabomber tends to strike on the samedates. But Harvard Police said they are notconcerned by the historical November bombings, andare not increasing security measures.

FBI officials said there is no need to panicand that professors and students should continueprevious precautionary practices.

"Just don't take chances," Smith said. "If youreceive a letter and it does not have a returnaddress, be cautious. You may wish to call yourlocal enforcement officer or postal inspector."

The average size of a mail bomb is usuallyeight by eleven inches, but they come in allshapes and sizes, Smith said