The deadly confrontation lasted less than a minute, road rage spilling into a Best Buy parking lot.
Lee Cena was angry -- William O'Brien had honked his horn too many times, officials say. So the two men scuffled outside the Fairfax, Va., store, an exchange captured on surveillance video.
After the April 2013 argument, officials say, O'Brien walked into the store and Cena drove away.
Within two weeks, O'Brien was dead.
Cena, 58, is in court this morning, facing misdemeanor assault and battery charges for his role in the fatal fistfight.
O'Brien, 63, started experiencing problems hours after the fight, officials say. He called 911, telling the dispatcher, "I have a headache that is about to make my head blow off."
William Houda, a medical expert, has said that O'Brien likely would have survived Cena's punch if he hadn't been taking prescription blood-thinning medicine, according to the Washington Post.
The case was so extraordinary that prosecutors were initially unsure how to charge Cena. He was initially charged with aggravated malicious wounding, a felony. Prosecutors later dropped that charge, and Cena was indicted for assault and battery.
Recently released video of Cena's police interrogation sheds new light on the confrontation, showing the moment Cena, who used to serve as director of religious education at a Fairfax church, first learned of O'Brien's injuries.
"You're kidding me," he said during the interrogation.
Cena admits following O'Brien into the parking lot, but says it was only to confront him verbally. He has pleaded not guilty to the charges against him.
"He was the one who was aggressive," Cena said during his interrogation. "I only struck him after he hit me."
Prosecutors say Cena threw the first punch.
Mark Eiglarsh, a former prosecutor in Miami with no connection to the case, said jurors are going to need to identify the fight's initial aggressor:
"Was it the defendant who went up to the victim and said some things verbally, or was it the victim, who an eyewitness claims was the physical aggressor, coming at the defendant?"
If convicted, Cena could face a year in jail.