Ben Carson, now surging to a national lead in the polls for the Republican nomination for president, has repeatedly told the story of one of his darkest moments: the time he attempted to stab a classmate in a fit of rage.

He referenced the incident most recently in response to Donald Trump’s accusation that Carson is “low-energy.”

“There was a time when I was, you know, very volatile,” Carson said. “But, you know, I changed.”

The story, as he told it this past week, goes something like this: Carson was 14 years old. He and his friend were arguing over radio stations. The disagreement escalated, Carson tried to stab him, and the blade ended up hitting his friend’s belt buckle—causing the knife to break and saving Carson and his victim from harm.

But the circumstances surrounding the failed stabbing have shifted in the 20 years since Carson began telling it.

The first time Carson shared this story was nearly two decades ago in his two books released in 1996. In the lesser known of the two, Think Big: Unleashing Your Potential for Excellence, Carson describes the tale as a seminal moment in his process of growing up.

“One other factor played an important role in my development. I had always had a terrible temper, striking out at anyone who opposed me. One afternoon when I was fourteen, I argued with a friend named Bob. Pulling out a camping knife, I lunged at my friend. The steel blade struck his metal belt buckle and snapped,” he wrote.

In Gifted Hands, the autobiography that thrust Carson into celebrity status, and spawned a miraculously awful made-for-television movie with the star of Snow Dogs, the story is much more detailed.

“I was in the ninth grade when the unthinkable happened. I lost control and tried to knife a friend. Bob and I were listening to a transistor radio when he flipped the dial to another station. [Note: In the film version of Gifted Hands, ‘Bob’ is annoyed that classical music is playing. A young Ben Carson is whittling at a table outside of school.]

‘You call that music?’ he demanded.

‘It’s better than what you like!’ I yelled back, grabbing for the dial.

‘Come on, Carson. You always—’

In that instant blind anger—pathological anger—took possession of me. Grabbing the camping knife I carried in my back pocket, I snapped it open and lunged for the boy who had been my friend. With all the power of my young muscles, I thrust the knife toward his belly. The knife hit the big, heavy ROTC buckle with such force that the blade snapped and dropped to the ground. I stared at the broken blade and went weak. I had almost killed him. I had almost killed my friend.”

Carson concludes the anecdote by saying he told Bob he was sorry and then ran home, locked himself in the bathroom, contemplated his actions, and found God in part due to the incident.

Fast-forward to Carson’s 2000 autobiography The Big Picture. In one chapter, Carson discusses a speech he gave in Baltimore to a “restless standing-room-only audience,” in which he once again told the saga of his knife attack.

“I told how I had gotten so angry one day that I lunged at a friend with a knife. I aimed at his stomach, but I hit his belt buckle instead. Rather than slicing open my friend’s abdomen, the blade broke off, and my friend ran away terrified but otherwise unhurt. Afterward, I was almost as frightened as my friend by the realization of what had almost happened. I could have very well ended up in jail instead of Yale. Instead, God used that incident to help turn my life around.”

In this retelling, it’s the friend who runs away instead of Carson.

The story comes up once again in Carson’s 2007 book Take the Risk, in which details return.

“One day, as a fourteen-year-old in ninth grade, I was hanging out at the house of my friend Bob, listening to his radio, when he suddenly leaned over and dialed the tuner to another station. I’d been enjoying the song playing on the first station, so I reached over and flipped it back. Bob switched stations again. Then something snapped inside of me. A wave of rage welled up, and almost without thinking, I pulled out the pocketknife I always carried. In what seemed like one continuous, involuntary motion, I flicked open the blade and lunged viciously, right at my friend’s stomach. Incredibly, the point of the knife struck Bob’s large metal belt buckle and the blade snapped off in my hands.”

In this instance, Carson is at Bob’s house. And the weapon, in this case, is also referred to as a “pocketknife” instead of a “camping knife”—which, this time, he pulls out of his pocket. In other iterations of the story, Carson already had the knife in his hand before the attack. In the film adaptation of Gifted Hands, for example, Carson is seen whittling a stick with a large hunting knife and playing classical music on the radio, which incites the other boy’s anger.

Perhaps the biggest departure from the original version of the story comes in 2011’s America the Beautiful, in which the situation is described much more as a random encounter.

“Because of the racial and socioeconomic injustice I experienced as a boy, in my anger and frustration I began to retaliate by going after people with baseball bats, rocks, and knives. One day a boy pushed me too far. I told him to back off, but he wouldn’t quit pestering me. Finally, I pulled out my knife and lunged at him, striking him in the abdomen. He fell back, and for a moment I thought I had killed him, but just then my knife blade fell to the ground. It had hit his belt buckle and snapped in two.”

In all retellings of this story, Carson gets angry, pulls out a knife, and the knife hits a belt buckle, saving his tormenter from harm. But, recently, pivotal parts of the story became very different.

In 2014, Carson released One Nation, and the story becomes less specific.

“I had been minding my own business when a classmate came along and began to ridicule me. I had a large camping knife in my hand and, without thinking, I lunged at him, plunging the knife into his abdomen. He backed off, certain that he had been mortally wounded before discovering that the knife blade had struck a large metal belt buckle under his clothing and broken. He fled in terror but I was even more terrified when realizing that I had almost killed someone. That incident led me to prayerfully consider my plight and to ask for God’s guidance and help. I came to understand that very day that I was always angry because I was selfish.”

In this iteration, both the encounter and his alleged bully are random, the knife—now not a pocketknife but a “large camping knife”—is already in his hand, the location is unclear, and it is now Carson’s victim—and not Carson, himself—who runs away.

Carson has released three books since then—One Vote in 2014, What I Believe, earlier this year and A More Perfect Union, for which he is on tour. There is no mention of the story in any context in those works.

Carson has been retelling the story in recent weeks to illustrate a moment of unbridled fury that led him to find his faith. He says it is, in part, the moment that transformed him from troubled teen to medical wunderkind—and turned him into the man of God he is today.

The Daily Beast has made repeated requests for Carson’s campaign, and specifically his business manager Armstrong Williams, to provide a retelling of the story and to clarify who Bob is and what role he played in Carson’s life.

The campaign has not responded to multiple requests for comment.