The case of Pvt. Lindsey Scott, the black marine who was convicted 16 months ago of raping, sodomizing, kidnaping and attempting to murder a white woman at the Quantico Marine base, has taken a dramatic turn.

For the first time, a witness testified last week that Scott, a 29-year-old husband and father whose service record was unmarred, was at a department store miles from where the victim was abducted.

The testimony came in a military court hearing to determine if Scott's original lawyer represented him effectively in his October 1983 court-martial. The trial attracted attention from civil rights groups, including the NAACP and the Southern Christian Leadership Conference, which charge that Scott was victimized by the military because of his race.

Military prosecutors, while convinced that Scott is guilty, acknowledge that the testimony last week weakens the case against Scott. He has maintained his innocence from the outset.

The new testimony strengthens the chance of a retrial, either on the orders of Lt. Gen. David M. Twomey, the base commander at Quantico, or the Navy-Marine Corps Court of Military Review in Washington. Both also have the authority to dismiss the charges against Scott, who is serving a sentence of 30 years' hard labor. Dismissal of charges is considered unlikely.

If a retrial is ordered, however, it is not certain that the victim, who now lives out of the state, will testify again. "She wants to put it behind her, she wants to forget it," said one Marine source who is close to the case but who insisted on anonymity.

With the case at a critical juncture, at least two elements stand unchanged. Scott, who was demoted from corporal to private, remains in the brig at Quantico. And questions about the evidence against him -- as well as the military justice system that found him guilty beyond a reasonable doubt -- remain.

The Scott case also brought allegations of racism from civil rights groups. The SCLC has scheduled a meeting today with Quantico officials to discuss racial attitudes at the base in Prince William County, 45 miles south of Washington.

In a letter after the court-martial to Rep. Ronald V. Dellums (D-Calif.), one SCLC official declared: "Scott was victimized in a hotbed of racial tension . . . . His chances of receiving a fair trial in such a racist environment were indeed poor."

Marine officials bristle at the charges. They say civil rights groups, nursing an unfounded suspicion of the military's racial attitudes in the first place, have used the Scott case as a vehicle for publicity.

Said one officer: "When that girl comes out of the woods and says her attacker is a black man, who would they have us investigate? A Hispanic? A Caucasian?"

Scott became a suspect almost immediately after the victim, the 23-year-old wife of a military policeman who worked with Scott, told investigators she had been lured from her apartment the night of April 20, 1983. The woman said she received a call from a man who said her husband had been in an accident. The man, who said he was from Quantico's Criminal Investigation Division -- Scott's unit -- met her downstairs minutes later. He said they were going to the hospital.

She said he drove her to a remote area on or near the base where he raped her, then took her from the car, choked her, stabbed her in the neck and slashed her throat with a knife. She said he laughed "in a weird way," then drove off. The victim was found wandering on the base that night, bleeding and dazed.

The woman identified Scott as her assailant twice, first from a group of photos, and then from a lineup, but both times said other men in the groups looked similar.

In neither session was she as certain as in the court-martial, when she said Scott "looked exactly like my attacker," according to testimony. She never mentioned Scott's prominent gold tooth.

In interviews with investigators while she was still in the hospital, however, she described the interior of Scott's car in detail. She knew his steering wheel was padded, that a rectangular ornament dangled from the rear view mirror, and she accurately described the precise location and type of gas gauge and ashtray.

She also remembered a metal bucket full of cleaning equipment in the car. Scott said he had a green plastic bucket of cleaning equipment.

When Prince William County police officers examined Scott's car a day after the assault, they said they found daubs of water and smear marks on the front passenger side of the car, where the rape allegedly occurred, indicating that it had recently been cleaned. Scott said at the court-martial the officers were lying.

One Marine officer close to the case noted that the investigators had the victim's positive identification of Scott and "an ironclad description of the guy's car" before they pressed charges. "It was strong enough to get a conviction beyond a reasonable doubt," he said. "We don't have to apologize."

None of the victim's fingerprints was found in Scott's car. Other forensic evidence, including samples of Scott's hair and body fluids, was either inconclusive or negative.

Prosecutors offered no conclusive medical proof to corroborate the woman's testimony that she was raped. No physical evidence emerged tying Scott to the crime, and there were no witnesses.

Prosecutors said Scott used a steak knife he borrowed from his landlord to stab the woman. Scott said he threw the knife away inadvertently after using it to clean his stove. No weapon was ever found.

Scott's lawyer in the court-martial, Ervan E. Kuhnke Jr. of Dumfries, tried to establish an alibi defense. Scott said that he spent the evening of the crime buying a gift for his wife and browsing at Zayre department store in Woodbridge.

Kuhnke said he had assumed that Cynthia Ausby, a former store detective at Zayre, would testify that she saw Scott in the store. But Ausby said at the court-martial she was unsure of the date.

Last week, Ausby reversed her testimony. She explained that she became certain of the date she saw Scott only after the court-martial when she checked store records of shoplifting arrests. She said she recalled following Scott that night after participating in two arrests.

If there is a retrial, prosecutors are likely to attack Ausby's credibility. "What she said now is going to be bounced off what she said before," said one Marine officer.

Critics of the military's handling of the case have also charged that top brass at Quantico signaled the all-military jury to convict Scott.

They cite the case of Lt. Col. R.T. Harry, the military magistrate who first heard Scott's case about two weeks after the crime. Harry found the evidence weak and ordered Scott released from jail. The next day, Harry was relieved of his assignment as a magistrate.

Marine spokesmen acknowledge the timing of the move was "atrocious." They said Harry had already requested retirement, and that there was no link between his decision to release Scott and his leaving the service.

The jury "knew exactly what happened to me," Harry said in an interview, referring to what he calls his firing. "And they didn't want it to happen to them. That's always the problem with military justice. It leaves a lot to be desired."