Surveillance video from a Best Buy store shows the altercation between Librado Cena and William O’Brien on April 16, 2013. The second clip shows Cena interrogated by Fairfax City Police Detective Michael Boone on April 18, 2013. (Fairfax City Police)

Surveillance video from a Best Buy store shows the altercation between Librado Cena and William O’Brien on April 16, 2013. The second clip shows Cena interrogated by Fairfax City Police Detective Michael Boone on April 18, 2013. (Fairfax City Police)

The driver behind him was honking “incessantly,” Librado Cena testified at his trial, so he pulled his minivan into a Fairfax City parking lot last April to yell at the man who had irked him so much.

Cena, 58, told jurors he confronted William O’Brien, 63, and the man turned and said, “Let’s go.” A scuffle ensued, and Cena testified he landed a single punch to O’Brien’s face. O’Brien died from the blow days later.

A Fairfax County jury convicted Cena on Tuesday of misdemeanor assault in the road rage incident that ended in an improbable tragedy. Cena faces a maximum of 12 months in jail when he is sentenced Wednesday.

Both families declined to comment after the trial, but during his testimony, Cena acknowledged the terrible pain and loss both had suffered and how easily it could have been avoided if he had simply kept driving. Cena and O’Brien lived miles from each other in Fairfax City.

“None of this would have happened,” Cena told the jury. “He would still be alive. I wouldn’t be here.”

The verdict followed a two-day trial that featured competing claims from prosecutors and the defense about who instigated the altercation. The fight was captured by surveillance cameras at the Fair City Mall and the images were played for the jury.

The incident began when Cena, a former director of religious education at a Fairfax church, told authorities he encountered O’Brien on Pickett Road near the mall. Cena testified that O’Brien, a veteran who was soon to become a grandfather, kept honking at him at every traffic light for an unknown reason.

Prosecutor Marc Birnbaum said Cena was at fault because he took the initiative to follow O’Brien into the parking lot on the morning of last April 16. The surveillance video shows Cena running up behind O’Brien. Birnbaum said that gave O’Brien reason to fear for his safety and defend himself.

“He’s angry. He’s mad. He’s upset. He’s irritated,” Birnbaum said of Cena in his closing argument. “He’s looking for a fight and he got what he was looking for.”

Defense attorney Kelly Sprissler said Cena intended only to tell O’Brien off, not scuffle with him. She said O’Brien threw the first punch and Cena punched back only in self-defense. She cited Quan Hoang, a witness who testified that O’Brien appeared to be the aggressor, although he saw only the latter portion of the fight.

“What happened on April 16 between these two men was a tragedy, not a crime,” Sprissler told the jury .

The encounter between Cena and O’Brien initially appeared to be minor and no one called police. O’Brien even finished running an errand afterward at Best Buy, but he called 911 from his home two hours later complaining about a headache that was about to make his head “blow off.”

He collapsed, and died 10 days later.

A medical examiner ruled O’Brien’s death a homicide and said the cause of death was complications related to traumatic brain injury from the blow.

William Houda, a doctor who was called by the defense to testify, said O’Brien was made vulnerable to a single punch by Pradaxa, the popular blood thinner he was taking. Houda said the drug prevents blood clotting, so bleeding continued in O’Brien’s cranium, causing his brain to be squeezed and damaged. He said that otherwise, O’Brien probably would have lived.

“I would not have expected that force of injury to cause a fatality,” Houda said of Cena’s punch.

Police originally charged Cena with aggravated malicious wounding, a felony that carries a maximum sentence of life in prison. But Fairfax County prosecutors dropped that charge, saying it didn’t fit the circumstances of the case. Eventually, a grand jury returned an indictment for misdemeanor assault and battery.

The sequence of events angered both families. O’Brien’s daughter was upset that Cena was not facing a more serious charge. Cena’s family said the seriousness of the initial charge and the publicity it created ruined his reputation. He was fired from his job and has been unable to find work.

At the trial Tuesday, the jury found Cena guilty of assault, not assault and battery. Assault is an act that creates the fear of physical harm in the victim, while battery is the act of physically touching someone in a harmful manner.

“All of us are thankful for the jury’s time and attention,” Sprissler said after the trial.

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