In early September 1983, a letter arrived in Lori Jackson's mailbox from Cpl. Lindsey Scott, United States Marine Corps, No. 407789524.

In 14 hand-written pages, Scott's letter recounted a sequence of events that had changed his life: A fellow marine's wife had been raped and her throat slashed, the subsequent investigation had focused on Scott, and in a few weeks he was to be court-martialed at Quantico Marine Corps base, about 35 miles south of Washington.

Scott had never met Jackson, a civil rights activist with a penchant for publicity and a knack for finding herself in front of television cameras, but she had become something of a celebrity in Northern Virginia. A few months earlier, Jackson had led a band of TV crews and reporters to Marshall, Va., where a local restaurateur had refused to serve blacks.

Scott, who described himself in the letter as a marine dedicated to the corps, to his family and to the Lord, implored Jackson to "come to my aid in this time of need," adding, "I could never rape or attempt to commit murder to no one."

By the time she finished reading the letter, Jackson was convinced the 27-year-old marine was innocent. From that moment, the case of United States v. Scott was transformed from a line entry on Quantico's judicial docket to her crusade for civil rights and individual justice in the Marine Corps.

Today, as Scott's second court-martial in five years nears its denouement, Jackson is recognized as Scott's most ardent patron -- in the words of Scott's attorney John F. Leino, "the heart and soul behind our case."

Scott's single best chance for acquittal -- the Zayre store detective who testified last week that she saw Scott shopping 12 miles from Quantico at the time the woman was abducted and raped -- is the result of Jackson's self-appointed involvement in the case. She found the detective -- five months after Navy investigators had overlooked her.

But after five years of fighting for Scott, of tracking leads and phoning reporters and rallying civil rights leaders, Jackson has been most notable by her absence from the spectator gallery in tiny Courtroom No. 1 at Quantico's Lejeune Hall since the current trial started Jan. 25.

The rumors that she has been diagnosed with cancer are not true, says Jackson, 50. It's just high blood pressure, she says; the doctor told her not to go near the courtroom, fearing the trial would upset her. "I heard a rumor, too," she said in an interview last week. "They had me on my death bed. Do I look like I'm dying?"

Nearly six feet tall, Jackson is a striking woman with a mop of auburn hair. She has lost a tremendous amount of weight since last summer. The whites of her eyes are yellow as lemons, and her weight loss has drawn the skin tightly over her cheekbones.

Jackson was admitted Sunday to the Washington Hospital Center. No details of her condition were available.

In conversation, she is a tireless raconteur, always sure of her direction if occasionally muddy on the details. Journalists who know her say that if she tends toward the melodramatic or is occasionally prone to exaggeration, she is nonetheless without malice, approaching her various crusades with a bon vivant's sense of adventure.

These days, she is eager to discuss the Scott case; her childhood brushes with the Ku Klux Klan in Decatur, Ala., ("I was not one of those mudpie-making girls"); her delight in befriending Germans after World War II when her husband was briefly stationed in Europe, and debunking the Nazi orthodoxy that blacks were subhuman.

Mostly, Jackson, who moved south of Fredericksburg from Dale City a year ago, is looking forward to taking a honeymoon in Hawaii. She didn't have time for one when she was remarried in 1983, she said, because that's when she heard about Lindsey Scott.

Jackson said Scott's letter, received eight days after her mother died, struck a chord: Jackson, who for years was the wife of an enlisted Army man, had sued the Army in 1971 when the Army tried to evict her from a military base in Kentucky after she pressed the base school to recognize black heritage as a legitimate field of study.

In a New York Times article, Army personnel officers at Fort Campbell, Ky., described Jackson as a trouble-causing black "militant." One compared her to a pimple, adding, "You remove it before it gets too sore." The same officer likened Jackson's husband, a veteran of Vietnam and Korea, to "a nigger caught in the wood pile." In the ensuing furor, the Army backed off its demand that Jackson and her family leave the base immediately.

For some time, Jackson said, her seven children delighted in calling her "Pimple" and making jokes about acne treatments, but the lesson of her tangle with the Army was clear: The military, like other U.S. institutions, was prone to racism, and would not change on its own without pressure.

From his letter, Scott sounded as if he was facing the same sort of bind that had entangled her 12 years earlier in Kentucky. "He felt like he was up against heavy guns," she said. "It was so similar . . . . It was like reliving a part of my life except I was not the main player."

Despite her lack of formal investigative training, Jackson assigned herself the role of chief investigator for Scott's defense, retracing the steps of Navy investigators who had checked Scott's alibi shortly after the woman was attacked.

Even though it was months after the crime had occurred, it was the first time anyone representing Scott had tried to corroborate his story -- that he had spent the evening shopping for his wife's birthday, which was the next day.

At the pharmacy, the ice cream stand and the department store where Scott said he had been the night the woman was attacked, Jackson cornered employees, flashing a booklet of photos of Scott. Did this man look familiar? Did they remember seeing him in the spring? Could they remember the date? The time? His clothing?

On her second visit to Zayre in Woodbridge, Jackson found what she was looking for. She asked the manager if any store detectives were on duty the night of April 20, 1983. And that's when she heard the name Cynthia Ausby.

Ausby said she had seen Scott. She testified she had watched him in the store, thinking he might be a shoplifter, but she couldn't be certain of the date. Jackson told her Scott's attorney, Ervan E. Kuhnke Jr., would get in touch with her.

He never did. Kuhnke, certain the government's case was anemic and Scott would not be tried, never prepared Ausby to testify, never spoke to her until moments before putting her on the stand, according to testimony in an appeal proceeding three years ago. He assumed she would provide the alibi Scott needed for an acquittal. Instead, Ausby waffled, still unsure of the date she had seen Scott.

Scott was convicted and sentenced to 30 years at hard labor. "I was devastated," said Jackson. "I was disappointed that an injustice like this could happen in the military. It was so obvious."

After the verdict was rendered, she fashioned a sign -- "Justice for Scott" -- and picketed the main gate at the Quantico base.

With a new team of defense attorneys, Jackson ran down dozens of tips in an effort to establish an alibi for Scott. Meanwhile, Ausby changed her story, saying that after checking store records, she was certain she had seen Scott on the evening of April 20, 1983 -- the day the woman was attacked.

Eventually, Ausby's new account contributed to a military appeals court decision last summer to overturn Scott's conviction. The decision was based on what the appeals court said was Kuhnke's failure to prepare a competent defense. Had Kuhnke prepared witnesses such as Ausby for testimony, the court reasoned, the outcome of the original trial might have been different.

The Marine Corps announced in the fall that Scott would be retried on the original charges: rape, attempted murder, sodomy and abduction.

When Ausby testified last week, prosecutors attacked her credibility, suggesting in cross-examination that she had been swayed by Jackson, hinting that Jackson had appealed to her concern for civil rights. Ausby, they said, had accompanied Jackson to the restaurant in Marshall where the owner refused to serve blacks. Weren't they friends? prosecutors asked.

Ausby denied her testimony was affected by Jackson and stuck by her story. Prosecutors are expected to continue assaults on her testimony with rebuttal witnesses this week.