Every crime-fighting superhero has a creation story. Nancy Grace, the prosecutor turned breakout star at CNN Headline News, has a particularly moving one. As she tells it, in the summer of 1980, she was a 19-year-old college student in small-town Georgia, engaged to Keith Griffin, a star third baseman for the Valdosta State University Blazers. The wedding was a few months away.

Then, one August morning, a stranger—a 24-year-old thug with a history of being on the wrong side of the law—accosted Griffin outside a convenience store. He shot him five times in the head and back, stole $35 from his wallet, and left him dead.

Police soon tracked down the killer, and a new phase of suffering began for Ms. Grace. The suspect brazenly denied any involvement. At trial, Ms. Grace testified, then waited as jury deliberations dragged on for three days. The district attorney asked her if she wanted the death penalty, and in a moment of youthful weakness, she said no. The verdict came back guilty—life in prison—and a string of appeals ensued.

For Nancy Grace, the ordeal she describes felt nothing like justice. And so the Shakespeare-loving teen set out to change the justice system: first as a bulldog prosecutor, then as a Court TV and CNN anchor, crusader for victims’ rights and professional vilifier of the criminal-defense industry.

Her message, delivered with a crackling blend of folksiness and wrath, has made her a hit on two cable networks. Defense attorneys are pigs—morally comparable, she said in a Feb. 20 interview with USA Today, to “guards at Auschwitz.” Her latest show, Nancy Grace, celebrated its first anniversary on CNN’s Headline News Network that week; in one year, its viewership has tripled, to 606,000 a night.

Because of what happened in Georgia, Ms. Grace has said over and over, she knows firsthand how the system favors hardened criminals over victims. It is the foundation of her judicial philosophy, her motivation in life, her casus belli.

And much of it isn’t true.

Nancy Grace was engaged to a man named Keith Griffin. He was murdered in Georgia. And the man who killed him is serving a life sentence. In that, Ms. Grace’s version lines up with the official records from the Georgia Bureau of Investigations, newspaper articles from the time of the murder, and interviews with many of those involved in the case.

But those same sources contradict Ms. Grace when it comes to other salient facts of the crime and the trial—the facts that form the basis of Ms. Grace’s crusade against an impotent, criminal-coddling legal system.

• Griffin was shot not by a random robber, but by a former co-worker.

• The killer, Tommy McCoy, was 19, not 24, and had no prior convictions.

• Mr. McCoy confessed to the crime the evening he was arrested.

• The jury convicted in a matter of hours, not days.

• Prosecutors asked for the death penalty, but didn’t get it, because Mr. McCoy was mildly retarded.

• Mr. McCoy never had an appeal; he filed a habeas application five years ago, and after a hearing it was rejected.

Ms. Grace has also misreported the date of the incident—it was in 1979, not 1980—and has given Griffin’s age as 25 when it was 23.

The justice system, in other words, apparently worked the way it was supposed to.

In an emotional phone interview ranging over the inconsistencies in her account, Ms. Grace said, “I have not researched the defendant. I have tried not to think about it.”

“She has bent some stuff,” said Steve Griffin, Keith Griffin’s brother, in an interview with The Observer. “The reality of it is, the guy killed him. I know that. Our family knows that. There’s nothing we can do to bring him back. What she’s gonna say, she’s gonna say. I’m not gonna stop it.

“But if she doesn’t tell the truth, it’s gonna come out sooner or later.”

Nancy Grace anchors three hours of live television a day: two during the afternoon on Court TV’s Closing Arguments, then an hour of Nancy Grace on Headline News in the evening.

Before Nancy Grace, Headline News was exactly as it sounded—a virtually uninterrupted news-reading circuit of the day’s top stories. Then Ms. Grace joined the primetime lineup, fresh off an intense run of commentator and guest-host spots on Larry King Live.

Ms. Grace, who will not discuss her age, but according to official records is 46, comes across as the Bill O’Reilly of legal analysis, shutting down dissidents and cozying up to the likeminded, whom she addresses collectively as “friend.” She is a self-identified advocate for victims’ rights, with a taste for cases of the missing-white-woman variety.

And she rarely lets a week pass without reminding viewers of her own history. During her Feb. 24 show, she brought it to bear on the spokesperson for Jennifer Hagel-Smith, a bride whose husband disappeared during their honeymoon cruise.

“Why does Jennifer Hagel-Smith need a P.R. person?” she asked. “I’m a crime victim. I didn’t need a P.R. person. Why does she need a P.R. person?”

Ms. Grace’s accusatory style makes for gripping, if not always judicious, television. (Regarding the Auschwitz quote, Ms. Grace offered an explanation on the phone: “Under no condition is a defense attorney equal to a Nazi guard. That’s just an extreme example of someone refusing to take responsibility.”) In February, Ms. Grace’s staff was ordered to attend a three-hour workshop on reporting basics to help remedy “lax journalistic standards,” according to a CNN source. The session covered issues such as the meaning of “off the record” and the number of sources it takes to confirm a piece of information.

“[E]very editorial employee is expected to attend these seminars, as part of CNN’s standard training,” CNN publicist Janine Iamunno wrote in an e-mail. Ms. Grace did not attend—because, Ms. Iamunno explained, “she had attended a previous seminar.”

In her previous career, as an assistant district attorney in Atlanta from 1987 to 1996, Ms. Grace was cited three times for sloppy trial practices. She argued hundreds of jury trials and never lost one—another chapter in her mythic back-story.

But in 2005, the 11th Circuit Court in Georgia declared that Ms. Grace had “played fast and loose” with facts in her 1990 triple-murder prosecution of Herbert Connell Stephens. In 1997, the Georgia Supreme Court overturned an arson-murder verdict, finding Ms. Grace had withheld evidence from the defense; in 1994, the same court had overturned her conviction of a heroin trafficker, finding problems with her closing argument.

Her courtroom style made a more positive impression, however, on Court TV founder Steven Brill. Mr. Brill plucked her out of an Atlanta courtroom in 1996. At his behest, as the story goes, she moved to New York with two suitcases, $200 and a curling iron, to co-host a show with Johnnie Cochran.

All along the road to fame, Ms. Grace has kept the focus on her original motivation.

“With every case that I prosecuted,” she told Tim Russert in June 2005, “every bad person I put away, it healed me. And looking back on it, I thought I was trying to help them, but I was really helping me.”

Through a spokesperson, CNN NewsGroup vice president Ken Jautz offered a written endorsement of Ms. Grace’s performance: “Nothing changes the fact that Nancy suffered a tremendous personal tragedy when she was 19 years old, with the murder of her fiancé—a trauma that shaped who she is today. While some details may be able to be clarified in the over 25 years since the case, they do not [bear] any significance on Nancy’s career as a prosecutor, victims’ advocate or television host. We have great respect for Nancy and her willingness to draw upon this personal experience as she advocates for victims and enlightens all of us on her show every night.”

“NANCY’S ALWAYS BEEN TENDERHEARTED,” said her mother, Elizabeth Grace. “She comes across as being so strong, but deep down, she has a lot of soft spots.”

By any standard, the murder of Keith Griffin shook Ms. Grace badly. She dropped out of college for a while, stopped eating and lost 30 pounds. She abandoned her plans to become an English professor and enrolled in Mercer University Law School, on her way to becoming a prosecutor.

Growing up, Elizabeth Grace said, her daughter had applied her tenacity to interior-design projects for her local 4-H club. She made little rooms out of particleboard and glued squares of carpet to the bottom. “She was always very competitive,” the elder Ms. Grace said.

Keith Griffin and Ms. Grace had been college sweethearts for more than two years when he proposed in the summer of 1979. Griffin planned to be a geologist, and was earning extra college money working for the Ingram Construction Co. on the Georgia Kraft Plywood Co. site near Madison, Ga. The engagement was a secret—only Griffin’s sister, Judy, knew about it—but the families approved of their relationship. Ms. Grace’s family thought Griffin was polite and charming; Griffin’s parents adored Ms. Grace. Griffin’s brother Steve, 13 months his junior, was the unimpressed one. “I thought she was a dingbat,” he said.

Then came Aug. 6. Griffin woke at 5 a.m. in the Grace home, where he’d spent the night in an extra room, and left for work. Ms. Grace’s mother said her daughter tucked some money in his hand before he left.

“I waved until he was nearly out of sight, because I’ve always heard that watching until someone is out of sight is bad luck,” Ms. Grace wrote in her autobiography, Objection!

At 8:30 that morning, Tommy McCoy, recently fired from his job at Ingram, went to his father’s house and took a pistol from the bedroom closet, according to the transcript of his confession given to two agents at the Morgan County Sheriff’s Department that evening. He wrapped the gun in a paper bag and hitched a ride with the family’s insurance man to his grandmother’s house, where he stayed until 11:15 a.m. Then he started walking toward Georgia Kraft.

WHAT HAPPENED TO KEITH GRIFFIN THAT DAY, Ms. Grace told Tim Russert in 2005, was this: “[H]e was, I guess you would say, mugged by someone—he didn’t even know him—and shot five times.”

“All this for $35?” Mr. Russert asked later in the interview.

“Thirty-five dollars,” Ms. Grace replied. “Thirty-five dollars.”

Ms. Grace also told Mr. Russert, “The man that murdered Keith was a repeat offender, and I thought the system had failed Keith.”

She had given the same message to a New York Times reporter in 2004: “The person that murdered Keith had several incidents with the law, and somebody let him slip through their fingers.”

And she told it to Larry King in 2003: “This perpetrator had been in and out of trouble. And I always wonder, if someone had cared about the case—not necessarily throw them behind bars and toss the key, but to rehab the person, or to throw them behind bars, to get him off the street.”

Nancy Grace’s stated mission has been to prevent that sort of mistake from being made again. “I am the system,” she declared to CNN’s Art Harris in a 1995 interview, when she was still a prosecutor. “I am part of the system, and it failed that time, and I hate to see it ever fail again.”

Yet it’s unclear when the system could have ever had a chance to lock up Mr. McCoy before the murder. According to his personal-history sheet, he had never been convicted of a crime. Ms. Grace noted in a written follow-up that Mr. McCoy could have had a sealed juvenile record.

On the phone, Ms. Grace said she recalled having been told “by an official” that “this young man had been in and out of trouble and that his own family had been afraid of him.” But she could not recall which official might have said that.

“I have not researched his background, no,” Ms. Grace said.

ACCORDING TO THE ATTORNEYS WHO TRIED THE CASE, as well as Mr. McCoy’s confession, the two men were not strangers, and nobody disputed who did the killing.

“He knew him, of course,” said Billy Prior, Mr. McCoy’s defense attorney. Griffin had seen Mr. McCoy walking and offered him a ride.

“[T]hat dude came up in a truck,” Mr. McCoy told police, according to the notes on his confession. “It was a blue truck, and it belongs to the man I used to work for. The boss wasn’t in it, and the guy that was in it was a white dude from Athens who worked with me for a while. He drove up next to where I was standing, and stopped the truck. He said, ‘Hello, Tommy, how are you?’”

Mr. McCoy then unloaded six rounds from his .38-caliber pistol. He took $10 from Griffin’s wallet and threw the wallet in the truck. The truck rolled into a ditch on the side of the road. Just then, Joe Brown, another employee of Ingram Construction, pulled up to see if Mr. McCoy needed help. Mr. McCoy trained the empty pistol on him, forced him out of his car, jumped in and drove off. “I shot that dude because he was one of the ones that got me fired from my job,” he said in his confession. “I went out there to get revenge because I had been fired.”

In her biography, Ms. Grace writes, “My deep-seated ethical problem with defense attorneys likely traces back to my being a witness in Keith’s murder trial …. The truth really doesn’t matter to the defense.”

In 2003, Ms. Grace told Larry King that the killer’s defense had been “Didn’t do it, wrong guy. Wrong place, wrong time.”

“No,” said Mr. Prior when asked about that account. “That was certainly not it.”

Mr. Prior laughed on hearing Ms. Grace’s quote comparing defense attorneys to Nazis. “I guess I’m where she got her idea,” he said. Mr. Prior is now a Superior Court judge in Morgan County and a fan of the Nancy Grace show.

“She’s not a reticent-type person,” he said. “I like that.”

But back in 1979, a mistaken-identity defense for Mr. McCoy would have had to overcome not only the confession but the testimony of Joe Brown, who had happened across the scene.

The most the defense could muster, Mr. Prior said, was a psychiatric evaluation, in which a doctor from the Georgia Central State Hospital declared Mr. McCoy “mildly retarded,” according to court documents.

“I don’t think I ever made a formal plea of insanity, because I couldn’t get a psychiatrist to say he was insane,” Mr. Prior said. “I did play on the mental-retardation thing. That was my only card.”

The district attorney who prosecuted the case, Joe Briley, is a longtime friend of Mr. Prior’s. Mr. Briley said he doesn’t care for Ms. Grace’s show. “I started to watch it one night,” he said, “but I don’t like the format. I knew right off that I didn’t like the format. I said, ‘I believe I’ll go watch a John Wayne movie or something.’”

Neither man recalled Nancy Grace as a pivotal figure at the trial. Mr. Briley faintly remembered asking her to identify Griffin’s wallet. “I don’t think she did anything to demark herself on the witness stand,” Mr. Briley said, “or I would have remembered.”

Ms. Grace offers vivid memories of the trial in her book. “The cavernous courtroom reminded me of the one in To Kill a Mockingbird,” she writes. She describes looking down at Tommy McCoy from the witness chair, some six feet off the ground.

And Ms. Grace’s mother recalls her coming home and recounting the trial point by point each night.

But Ms. Grace said that she has no recollection of Mr. McCoy’s confession or his defense strategy.

“I did not hear his defense,” she said. “I only recall my testimony.”

Where did she get the claim that Mr. McCoy had denied the killing? “Upon his arrest, I was told he said he didn’t do it,” Ms. Grace said. “He may have confessed at some juncture, but I was told that he initially said he didn’t do it.”

Columbus Johnson, the deputy sheriff who arrested Mr. McCoy and took him to jail in 1979, is now a captain in his 34th year with the department. “He didn’t open his mouth the whole way,” Mr. Johnson said. “He never said anything to me or the other officers that transported him.”

Asked if she had checked her memory against the official documents before writing the book and giving the interviews, Ms. Grace said, “I wrote about everything with the knowledge I had.”

The trial wrapped up quickly—not with three days of deliberations, as Ms. Grace described, but with one. Mr. McCoy was convicted of aggravated assault and murder, and acquitted of robbery.

Mr. Briley said he had no recollection of Ms. Grace’s dramatic, now-regretted decision not to ask for the death penalty in the sentencing phase. Rather than asking the family if they wanted to pursue death, he said, his practice was to tell them his plan and see if they supported it. “If she had been introduced by the family, and this may have been what happened, as his fiancée, then I would have included her in the conversation,” he said.

In any event, Mr. Briley did put in for the death penalty, in a letter on Oct. 3, 1979—saying the murder “was outrageously wanton or vile, horrible or inhuman in that it involved torture, depravity of mind, or an aggravated battery to the victim.”

The jury recommended life in prison. Both attorneys said they believe this to have been because Mr. McCoy was demonstrably slow-witted. “He was not very bright,” Mr. Briley said.

When the verdict was read, Mr. McCoy turned to Mr. Prior. “He asked me, ‘What does it mean?’” Mr. Prior said. “I told him it meant he was not going to the electric chair.”

Mr. McCoy has been in prison for 27 years now. “He did not appeal his case,” Mr. Prior said. “His family did not want to appeal.”

Five years ago, Mr. McCoy filed a habeas application with the state. “It’s not an appeal,” Mr. Prior said. “It’s a writ claiming he’s being illegally held.” Mr. Prior testified at a hearing, and the application was denied. He appears likely to remain behind bars for the rest of his life.

Ms. Grace, meanwhile, plans to continue her crusade. Even if the facts don’t exactly line up. Even if it’s starting to worry her mother.

“Like I told her so many times,” the elder Ms. Grace said: “‘Nancy, let it go.’”