ELEVEN YEARS AGO, the wife and two small children of Green Beret captain Dr. Jeffrey R. MacDonald were stabbed and bludgeoned to death in their home on the Fort Bragg, N.C., military reservation. MacDonald, also stabbed but well enough to talk, gave military investigators an accounting of what had happened in those early morning hours of Feb. 17, 1970, and a description of four assailants he said invaded his home.

The one he remembered best was a woman in a floppy hat, standing over him as he was being beaten, chanting, "Kill the pigs. Acid is groovy." The word "Pig" was written in blood on the headboard in the couple's bedroom.

Nine years later, when MacDonald went on trial for the murders, the government said the woman in the floppy hat was a fiction, the idea of a hippy cult murder an invention modeled on the then recent Sharon Tate slaying. The jury didn't believe MacDonald either. He was convicted and sentenced to three consecutive life terms. He was serving his time when the Fourth U.S. Circuit Court of Appeals reversed his conviction last year because of delays in bringing him to trial and set him free while the government appealed to the U.S. Supreme Court.

Now a person claiming to be the woman in the floppy hat has surfaced and says she was in MacDonald's house the night of the murders. She says she was the one who said "acid is groovy" as her friends, part of a drug-crazed cult, rampaged through the house beating, stabbing and terrorizing. MacDonald, she says, didn't do it.

It is, of course, not that simple. Nothing about the MacDonald case -- one of the most celebrated murder proceedings of its times -- is that simple. The woman had surfaced before, notably at MacDonald's 1979 trial. Lawyers tracked her down, rushed her to Raleigh and put her on the stand for what they thought would be one of those dramatic moments generally reserved for television scripts, a moment when a hushed courtroom, gasping in astonishment, would hear her exonerate the defendant.

Instead, she said she remembered nothing, nothing at all.

"Her memory," the court of appeals said when it reversed MacDonald's conviction, "has resembled a lightbulb not screwed tight, blinking on and off."

But now, in affidavits obtained by a private investigator hired by friends of MacDonald, in interview with The Fayetteville Times and The Washington Post, it blinks on. She says she remembers. She says she has had nightmares for years about it and after being tracked down by a private investigator, felt it was time to talk.

Government lawyers will not interview her and, to the anger of MacDonald's friends and attorney, have moved forward with their appeal which could send MacDonald back to prison. The government still believes that the woman's story is a fiction.

Fiction or not, Helena Stoeckley Davis still haunts this marathon murder case.

Before her story is told, it is worth repeating: Nothing about the MacDonald case is simple. Only the aficionados -- there are by now quite a few who have devoted the better part of a decade to it -- can keep everything straight. It is also important to know that among everyone involved, there are high stakes and high emotions and a lot of ego on the line. There is an unofficial club -- call it "friends of Jeff MacDonald" -- of people who believe deeply in his innocence and financed the private eye's successful search for the woman in the floppy hat. One of them sought this story in The Washington Post.

The private investigator, Ted Gunderson, a retired head of the FBI in Los Angeles, wants a man he considers innocent vindicated. But he also knows it would be a triumph for him and would help his business. Gunderson helped make the woman available to The Post for an interview. A friend of his, who is also a friend of Stoeckley's and a believer in MacDonald's innocence, was present during the interview but asked not to be identified.

MacDonald wants to be exonerated and continue his life in California. He is 38, good looking, a doctor. He has tasted prison life and prefers his condominium and boat in California. MacDonald's lawyers used the woman's statement in a private effort to convince the government not to appeal the reversal of MacDonald's conviction. Having failed, they want the Supreme Court to let stand the lower court's decision and want the justices to know that if they reverse it, they may be sending an innocent man back to prison. Beyond that, they had developed a strong, personal and passionate antipathy to the government lawyer in charge of the case.

The woman, Helena Stoeckley Davis, 29, was promised relocation, a possible job in California and a "new identity" for her cooperation with the private investigator. She has not claimed any of these and it seems she has more to lose than to gain: She is subjecting herself to a possible perjury charge, not to mention admitting being, at the least, an accessory to murder.

The government lawyer, Justice Department attorney Brian Murtagh, is also driven. He started on the case when he was in the military. At 34, he has invested much of his career in this one case. He says, with emotion, that he is not just representing the government. He is representing Colette, 26, Kimberly, 5, and Kristin MacDonald, 2, the victims.

Fueling the emotion has been the tumultuous history of the investigation itself. The crime scene was chaos. Army Investigators, for example, allowed local trash collectors to haul off trash receptacles outside the house.

The hospital threw out MacDonald's pajama bottoms, potentially crucial evidence. A medic allegedly stole MacDonald's wallet.

First the Army charged MacDonald. Then it exonerated him. Then, under intense Lobbying from MacDonald's father-in-law and congressional pressure, the Justice Department reopened the investigation in 1971. Then, four years after the crime, MacDonald was indicted. Nine years after the events, he was tried and convicted.

The prosecution was based on circumstantial evidence, physical evidence like blood stains, a bloody footprint, thread from MacDonald's pajama tops, from which Murtagh and his aides put together a two-pronged theory which was convincing to the jury.

The government first demonstrated to the jury's satisfaction that MacDonald's version of what happened could not have happened. Then the government put together its own hypothesis of what did happen:

MacDonald's marriage, prosecutors said, was under strain. MacDonald argued with his wife at 2 a.m. that morning after one of the children sleeping in the couple's bed wet it.The argument turned violent and bloody. One of the children got caught in the middle of the fight and was killed. MacDonald then attacked the second child, sleeping in her bed, as well. Then, the government argued, he arranged things to make it look like intruders, hippies in the Manson mold, had done it.

That was a summary of the prosecution's case.Here is a summary of the same case, as told by Helena Stoeckley Davis to private investigator Ted Gunderson last winter. In an interview with The Post, Davis confirmed her statement to Gunderson.

Fayetteville, N.C., where the MacDonald story began 11 years ago, was in those Vietnam war days at least two separate nations. There was Fort Bragg, the Army base and Green Beret training center where all-American boys with military haircuts trained before going into war. And there was another part of town where some of them would return, lucky to be alive but hopelessly hooked on bitterness or drugs or both. There they would mingle with locals, many the children of the military, who for reasons of their own, lived drug-saturated lives.

Jeff MacDonald, Princeton graduate, was part of the straight world. Helena Stoeckley, now Helena Davis, the daughter of a lieutenant colonel, was part of the other.

In February 1970, Colette MacDonald was expecting her third child. Jeff MacDonald was planning to finish his tour of duty as a Green Beret doctor and go off to Yale for advanced medical training. Helena Stoeckley, then 18, was working off and on, studying witchcraft and hanging out with a group she now describe as a drug-crazed cult. At one point, she worked at a pizza joint and, along with the pepperoni, served drugs to select customers. She says a fast-food place -- the House of Treats -- came to be known as the "House of Freaks" after she started working there. She was also, at the time, a regular drug informant for a Fayetteville police department named Prince E. Beasley.

She told Gunderson that she and at least five drug-using friends whom she described as part of a 13-member "cult" were "upset with Dr. MacDonald because he refused to treat heroin- and opium-addicted persons. . . . Approximately two weeks prior [to the murders] the leaders of our satanic cult met and began discussing the possibility of retailiatory acts against Dr. MacDonald . . .

"The leaders of the cult had previously decided to annihilate the MacDonald family, meaning murder them, except for Dr. MacDonald if Dr. MacDonald refused to furnish them with prescriptions for drugs or the drugs themselves."

She says she made a phone call to MacDonald's house early in the evening of Feb. 16 to check on the family's whereabouts. She said Colette answered, said there would be a babysitter at the home in the early evening but that after that the family would be there. a

After midnight, she said, the group engaged in a drug ritual. She "dropped mescaline." Then they drove to the MacDonald neighborhood, where five of them got out of the car, made their way to the MacDonald home and entered, apparently without a problem. She says she was carrying a lighted candle.

MacDonald was lying on a couch in the living room. "Everyone proceeded to the couch to talk to Dr. Macdonald. I recall hearing Collette say, 'Jeff, why are you letting them to this to me?'"

"Collette sounded like she was gurgling. I never heard her yell out again after that. It was kind of a hysterical yell like she was trying to clear her throat."

She said some of the group tried to talk to MacDonald about signing a prescription for dexedrine. "He became belligerent and hostile . . . . He started to fight in self-defense," but then asked to use the phone in an effort to get some drugs for them. Instead, she said, he tried to call the military police and "the situation was out of control."

The men started beating MacDonald. One had something in his hand and "when he drew back there was blood . . . As Dr. MacDonald started to lapse into unconsciousness, I yelled, 'Acid is groovy, kill the pigs.' Then I paused and yelled, 'Hit him again.' . .

"I remember running into the master bedroom . . . The first thing I noticed was the headboard of the bed. Written in blood was 'Death to All Pigs' or something of that nature . . . I realized that two people were bludgeoning Colette. She was on the bed. I saw one of the children asleep next to Colette on the bed. . . . It seemed like blood was everywhere." Amid this scene, someone called on the telephone, she says. Davis answered and was told by the others to hang up.

"During this ordeal, I wore a beige, floppy hat . . . "

The story, if true, would exonerate MacDonald, though it is clearly not a complete account. Davis says she never saw the children attacked.

Ted Gunderson, the investigator, has accumulated four volumes of information which he believes supports her story. Here are some of the points he makes:

It conforms in most respects with MacDonald's version of events given right after the murders. He remembers the woman in the floppy hat. He remembers "acid is groovy" and other comments Davis says she made. He also said he remembered a candlelike flickering light held by one assailant.

Wax-drippings were found in the house and tests found that they did not come from any candles then in the MacDonald house.

One of the military investigators arriving on the scene that morning did see a woman in a floppy hat waiting on a street corner.

MacDonald's descriptions of the men attacking her fit Davis' friends at the time and the people she was with when Beasley stopped her the next day.

A number of Davis' friends recall her telling them in the days that followed that she was involved in a murder.

While she could have constructed much of her story from publicized information about the case, Gunderson says some of her information has not been made public before, like the presence of a German shepherd in a yard near the MacDonald house.

An aide to Gunderson took a statement from an inmate saying he had called the MacDonald house that morning. But by the time Gunderson showed up to formally interview the man, he had escaped from work-release.

Gunderson had a lie detector test administered to her, which she passed.

There are also problems with her story.Among them:

MacDonald never remembered any discussions about prescriptions before he was attacked. He just remembers being attacked.

MacDonald said he had moved the child out of the master bedroom before he went to the couch (because his side of the bed was wet). Davis says the child was in Colette's bed.

No one has previously mentioned any plan to have a babysitter come to the MacDonald home that evening.

The hat and other pieces of potential evidence are nowhere to be found. Davis says she later disposed of them.

Her story does not change any of the physical evidence presented at the trial on which the jury based its verdict of guilty.

She admits to having been drugged during the entire period of the murders.

On each side, there are many more arguments. But most of the questions are for Davis to answer. Why now? Why did she say under oath on the witness stand that she couldn't remember anything? Davis acknowledges that she concealed information under oath at the MacDonald trial. After numerous vague answers, MacDonald's lawyer asked her if she had "a specific recollection" of where she was at the time of the murders.

"No, sir," she answered.

The U.S. district judge presiding over the trial would later describe her appearance this way: "The court gained the unmistakable impression which it believes was shared by the jury that this pathetic figure was suffering from drug-induced mental distortion and that she could be of no help to either side in the case."

In the Post interview, Davis said that at the time of the trial she did indeed have "glimpses," though only vague glimpses, of being present during the MacDonald murders. She said she concealed those recollections for several reasons. (The Court of Appeals cited Davis' inability to remember anything as a reason MacDonald's defense might have been prejudiced by the lack of a speedy trial. Perhaps, the court said, she would have remembered more had the trial been closer to the time of the murders.)

She did not, of course, want to go to jail for murder. She said she saw friends of hers in the courtroom and was worried about retribution if she told the truth. She said she resented being depicted by the prosecution as "a mental case, some kind of psycho." She said her cloudy answers about "glimpses" would have contributed to that image.

She also said she was at the time under the (mistaken) impression that MacDonald himself was responsible for reopening the investigation after the Army had exonerated him. He was free, she thought, but nevertheless subjected her and her friends to possible prosecution. "Every once in a while while I was up on the stand, I looked at Jeff. I thought how much I hated him. He was sitting there with this cocky grin on his face. He was just sitting there."

Now comes the debate about what to do with Davis and what to do with Gunderson's report. To MacDonald and his lawyer, Bernard L. Segal, the government should at least investigate the case further, talk to Davis and try to track down those she says were involved with her. The material at least raises a substantial "reasonalbe doubt" about MacDonald's guilt, he says.

"This is the job the government should have done," said Segal. "There are a tremendous number of leads in Ted's report, all coming out of one man's work. How much does one human being have to do? They [the Justice Department] ought to bury their faces in their hands, in shame," Segal said, almost sputtering with anger.

"I have reviewed it [the Gunderson report]," Justice Department prosecutor Murtagh said. "As an attorney who has lived with this for the last nine years. I find nothing in her statements to challenge the correctness of the jury verdict . . . I'm not going to throw in the towel" over this.

If the Supreme Court overturns the lower court decision and there is a possibility of a new trial or further proceedings, Murtagh said he would then consider a government investigation of Gunderson's findings.

If the court declines to review the case, MacDonald will remain an innocent man. He cannot be tried again on these charges by the federal government