Edward T. Mann, 41, who killed three persons and wounded six at IBM's Bethesda office building in 1982 in a terrifying attack in which he fired more than 130 shots, hanged himself yesterday in his cell in the Maryland Penitentiary in Baltimore, authorities said.

Mann, a Prince George's County resident who had at one time asked to be given the death penalty and was described as mentally ill by his defense attorneys, had been sentenced to three life terms plus 1,080 years in prison.

The former IBM employe was found by a guard about 3:15 p.m. hanging from a bed sheet that was attached to part of his cell's door housing, according to Department of Corrections spokeswoman Beverly Marable.

She said Mann left no note and that authorities had no idea why he would have killed himself.

At one point after the May 28 shootings, Mann was declared incompetent to stand trial and sent to a mental hospital where he went on a hunger strike and assaulted two psychiatrists. A psychiatrist who testified in his behalf said Mann believed himself a victim of "a global conspiracy centered on IBM."

Mann's death was discovered about five hours after the death of another inmate, who also appeared to have killed himself, Marable said. She said the other inmate was a 30-year-old Prince George's County man whose name was being withheld until his relatives could be notified.

Marable said the other man also had hanged himself with a bed sheet. But she said the other inmate was being kept apart from the general population of the prison in another wing of the prison and that she doubted that there was any connection between the incidents.

Both deaths are under investigation, and autopsies will be conducted, she said.

Marable said Mann was housed in a one-man cell among the penitentiary's general population. She said he had been at the penitentiary since shortly after his sentencing for the attack at the three-story IBM building on Fernwood Road, where he once had worked.

After Mann's surrender at the end of a 7 1/2-hour siege, authorities recovered five weapons, including a machine gun, from an office where he was hiding. They also found more than 250 rounds of unused ammunition.

Mann had been an IBM employe for 13 years, but, according to his wife, he quit because he was not advancing. In 1977 the former marketing representative argued unsuccessfully that he had been denied rapid promotions because he was black.

After his arrest Mann smashed a television set at the Montgomery County Detention Center and drank cleaning fluid. He spoke of conspiracies against him, praised his prosecutors and asked for the death penalty.

Declared incompetent to stand trial on Feb. 9, 1983, he was sent to Clifton T. Perkins hospital in Jessup, Md. Declared competent the next year after treatment at the hospital, he finally pleaded guilty to all 75 charges brought against him.

Defense lawyers contended that Mann's mental illness led him to reject an insanity defense and to enter the guilty plea.

At his sentencing in Montgomery County Circuit Court on Aug. 6, 1984, Mann spoke for more than an hour, asserting that he was "truly sorry" about the tragedy but showing few other indications of remorse.

Judge William J. Miller, in imposing the maximum possible sentence, told Mann that his statement, "like the crimes you committed, evidences a twisted mind."

The judge said he found it "almost impossible to fathom how such an intelligent and well-educated man can commit such terrible crimes and fail to beg the court for forgiveness."