NASHVILLE, March 7, 1982— New but long-held secret information was disclosed today in one of the most disputed trials in American history, the murder conviction and subsequent mob lynching of Leo Frank almost 70 years ago. (ed: episode occurred in the period 1913-1915)

Mr. Frank, a 29-year-old Jewish factory superintendent, was convicted in Atlanta of killing one of his employees, Mary Phagan, 14, and dumping her in the basement of the pencil factory where they worked.

But in a sworn statement to the newspaper The Tennessean, an 83-year-old Virginian who, seven decades ago, was a frightened and reluctant teen-age witness in that trial, now says that he saw the real killer bear-hugging the long-haired girl at her waist and carrying her limp, unconscious body to a partly opened trap door leading to the basement on the day she was murdered.

”Leo Frank did not kill Mary Phagan,” Alonzo Mann insisted in confirmation of a widely believed theory of historians. Says Janitor Was Murderer

”She was murdered instead by Jim Conley,” he asserted, referring to a factory janitor who was the chief witness against Mr. Frank. Mr. Mann was 14 years old at the time of the murder and was working as Mr. Frank’s office boy for $8 a week. He said the janitor, startled by the boy, threatened to kill him if he ever mentioned what he had seen that day.

Young Alonzo Mann was called to testify at the trial, but was asked only a few perfunctory questions. On the advice of his mother, he volunteered no information and told no one in authority what he had seen that Saturday, April 26, 1913. He said he continued to heed that advice for several years, except for an occasional confidence to relatives and a rebuffed attempt to tell an Atlanta newspaper reporter 30 years ago.

In later years, he said, he would have agreed, even been eager to talk with those who have written some 50 books about events surrounding the infamous trial. None of the authors ever approached him, he said.

But when confronted by two Tennessean reporters, Jerry Thompson and Robert Sherborne, who were acting on a tip, he related his story and supplied them with notes, photographs and other materials. He submitted to a lie-detector test and a psychological stress evaluation and passed both impressively, The Tennessean said. The newspaper reported that a two-month investigation satisfied it of the historical accuracy of his information and the validity of his claims.

”Many times I wanted to get it out of my heart,” the white-haired Mr. Mann said in an interview last night. ”I’m glad I’ve told it all. I’ve been living with it for a long time. I feel a certain amount of freedom now. I just hope it does some good.”

His lips trembled, but his clear blue eyes belied his frail physique and failing heart that pumps with the aid of a surgically implanted pacemaker. He now lives in Bristol, Va., where he said he is fond of his friends and his church.

”I know I don’t have a long time to live,” he said. ”All I have said is the truth. When my time comes, I hope that God understands me better for telling it. That’s what matters most.”

But he is reluctant to tell it again. ”I have laid that burden down and I don’t ever want to pick it up again,” he said. According to Mr. Mann’s account: He was working with Mr. Frank in the office that Saturday morning. He had encountered Mr. Conley early that day when the burly black janitor asked to borrow a dime for beer. Mr. Mann, who is white, did not lend him the money.

He worked until about 11:30 A.M., when he left the National Pencil Company factory to meet his mother to watch the Confederate Memorial Day parade. His mother did not show up, and he returned to work. Startled by Sight

He stepped into the first-floor vestibule and walked toward the stairs to the second floor. But a movement in the shadows caught his attention. He was riveted by the scene before him: Jim Conley standing beside a trap door clutching the wilted body of a young white girl. Her head lolled on the man’s right shoulder. She seemed either dead or unconscious. He saw no blood, no wounds, no rope.

The janitor looked over his left shoulder at the boy and their eyes locked. They faced each other for a few moments before the man spoke. ”If you ever mention this, I’ll kill you,” he said.

The frightened youngster fled back to the front door and ran outside. On arriving home, he told his mother what he had seen and recalled her saying: ”For God’s sake, don’t tell anybody else about this. You just stay out of it.”

Early the next morning, a night watchman found Mary Phagan’s bruised body lying face down in a pile of wood shavings in the basement. A ligature was around her neck, having scorched her throat. Blood had flowed from a deep cut in her scalp. There were signs of a struggle to escape. Her underclothing was ripped, but there was no evidence that she had been raped.

While Mr. Mann was out, she had come to the factory to pick up her pay, $1.20 for 10 hours work. Neither her purse nor her $1.20 was ever found.

The janitor accused Mr. Frank of the killing and said the defendant had paid him $200 to carry the body to the basement and burn it in the furnace.

Although nothing in Mr. Frank’s history indicated wrongdoing, no other evidence supported the janitor, and the prosecution acknowledged that Mr. Conley had told several other versions of his story, the community, and the jury were quick to convict Mr. Frank.

A wave of anti-Semitism was washing over Georgia and mobs swarmed the courthouse daily, screaming, ”Kill the Jew.” A local newspaper defamed Mr. Frank as ”a Jew Sodomite.”

Mr. Frank was sentenced to hang. He appealed his case unsuccessfully for two years before Gov. John Slaton commuted the sentence to life in prison days before his term expired in 1915.

The commutation produced a furor of protest. Armed mobs roamed streets forcing Jewish businessmen to board up windows and doors. A mob of several thousand people armed with guns, hatchets, and dynamite surrounded the Governor’s mansion until they were dispersed by state militiamen.

Within days of the commutation, 75 men calling themselves Knights of Mary Phagan met at the girl’s grave in Marietta, Ga., north of Atlanta, and vowed to avenge her death. They armed themselves and stormed a prison farm where Mr. Frank was being held 175 miles to the southeast in Milledgeville, and where he had survived a throat slashing by an inmate a month earlier.

The mob handcuffed Mr. Frank and transported him back to Marietta and hanged him from a oak tree a stone’s throw from Miss Phagan’s birthplace.

In the aftermath of terror, about half the 3,000 Jews in Georgia left the state. Those who remained hid behind locked doors, forced to survive a widespread boycott of Jewish businesses.

The Frank trial marked the rebirth of a moribund Ku Klux Klan movement that grew out of the Knights of Mary Phagan, and it also gave rise to the formation of the Anti-Defamation League of B’nai B’rith.

Recently Mr. Mann went back to Georgia and visited the grave of Mary Phagan and mused about the possibility that he could have saved her life, as well as Mr. Frank’s, if he had shouted out that Saturday in 1913. But he said he often had harbored such thoughts.

”Thousands of times I’ve gone to bed at night with all this on my mind,” he said Saturday. ”I hope you folks tell the whole world what I saw and that Leo Frank was innocent.”