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)

Librado Cena wheeled his gold minivan into the parking lot of a Fairfax City mall in April, intent on yelling at the driver who had repeatedly honked at him.

The Sunday school teacher, 58, confronted the soon-to-be grandfather, William O’Brien, and a scuffle ensued, the kind of awkward encounter that sometimes occurs between men unaccustomed to fighting. Cena later said he landed just a single blow to O’Brien’s head.

The altercation appeared so minor that no one called police and O’Brien popped into Best Buy to make a purchase. But about an hour afterward, the 63-year-old collapsed and never recovered. He later died.

Cena will stand trial Monday in a case that began as so many run-ins do on the Washington area’s congested roads but that turned improbably tragic. That blow has divided the families of the popular and well-respected men from Fairfax City over the legal conundrum at the heart of the case: Was it murder?

A medical examiner ruled O’Brien’s death a homicide, but Cena stands accused of misdemeanor assault and battery. That contradiction points to the extraordinary circumstances of O’Brien’s death and the little-known and sometimes fatal issue with a popular anti-stroke drug that made him vulnerable to a single punch.

Cena’s wife, Janice, said that her husband was defending himself and that his reputation has been “totally smeared” by the legal proceedings and news media coverage of the case. O’Brien’s daughter said Cena is getting away with a slap on the wrist.

“He loved to talk to people. He loved to entertain. He was killed just running an errand,” Kelly O’Brien said of her father. “I don’t have much faith in our justice system.”

A chance encounter

Cena and O’Brien lived just miles apart, putting down roots and raising families. Cena was the religious director of St. Leo the Great Catholic Church, which is the axis of his family’s life. All four of his children attended school there, and some answered the rectory’s phones.

O’Brien, a veteran and a retired mortgage industry worker, was well known for his love of his Irish heritage. He attended Celtic festivals in an O’Brien clan tartan and enjoyed telling people the history of their Irish names. His lavish St. Patrick’s Day parties featured a bagpiper. O’Brien had two grown children.

The pair had never met before they crossed paths driving on Pickett Road near the Fair City Mall on the morning of April 16.

Cena told police during an interrogation after his arrest that O’Brien began honking at him at every light, even though Cena said he had done nothing wrong. Cena would later acknowledge that he should have kept driving but that he was so enraged, he followed O’Brien’s car into the mall parking lot.

Surveillance cameras from Best Buy captured the entire encounter but no sound. It would last about 40 seconds.

Cena is seen running up behind O’Brien as he trudges toward the Best Buy. It is difficult to make out what happens on the video, but police said it shows Cena striking O’Brien on the back of the head, sparking the scuffle.

Cena denies investigators’ version of events.

He told police that he intended only to tell O’Brien off. By his account to authorities, he told O’Brien: “You don’t have to honk at me like that. One of these days you’re gonna get your ass kicked.”

Cena said he was surprised when O’Brien turned around and challenged him. “Well let’s go right now,” Cena said O’Brien told him.

Cena said that he began backing off but that O’Brien punched him in the face. In turn, Cena told police, he punched O’Brien in the face and then held his arms. Cena said he also knocked O’Brien’s glasses off. The defense contends that Cena punched O’Brien only once.

The video shows Cena moving backward across the parking lot as O’Brien moves forward. Quan Hoang, a lawyer, told police that he saw the latter part of the fight, saying, “The old man was more, you know, fighting.”

In the end, Cena told police, O’Brien scooped his glasses off the ground. He’s seen on the video strolling toward the Best Buy. He went inside and purchased a router just as he had planned, but the damage was done.

How a punch turned deadly

The punch had caused bleeding on O’Brien’s brain, a doctor who treated him told police. His cranium was slowly filling with blood, squeezing the delicate tissue of his brain.

O’Brien was in distress when he called 911 from his home about an hour later. He pleaded with the dispatcher for help: “I have a headache that, it’s about to make my head blow off.”

He collapsed on his dining room floor before paramedics arrived.

A medical expert is expected to testify at Cena’s trial that a sometimes deadly problem with an otherwise safe and popular anti-stroke drug O’Brien was taking contributed to his death.

Pradaxa keeps blood from clotting, so even a minor brain trauma — such as one from tripping and falling — can cause bleeding that will continue unabated. And doctors are largely powerless to stop it.

“We don’t have a good way to control it,” said Richard Schmidt, a professor at the University of Utah School of Medicine who co-authored a paper on the topic.

As O’Brien clung to life in the hospital, police arrested Cena and charged him with aggravated malicious wounding, a felony that carries a maximum sentence of life in prison and signals that authorities think the suspect intended to maim or kill.

O’Brien died 10 days after the clash. Kelly O’Brien said that her father was given a proper Irish wake and that the same bagpiper who performed at so many of his St. Patrick’s Day parties played “Danny Boy.” His family substituted “Willie” in the chorus.

She said one of the hardest parts was that her father passed away just two months short of the birth of his grandson. She said he made sure to call her after every checkup while she was pregnant. She named her son after him.

“I always tell my son how proud his grandfather would be of him,” O’Brien said.

Two families, one tragedy

The Cenas said they went to St. Leo’s to pray the night they learned that O’Brien had died. Kelly Sprissler, Cena’s attorney, said she thought around that time that police were going to increase the charge to murder.

O’Brien’s death drew a flurry of news media coverage in The Washington Post and elsewhere with headlines blaring “road rage.” Janice Cena said the serious charge and the coverage destroyed her husband’s reputation and hung a cloud over the family.

Librado Cena, who is known as Lee, lost his job as the religious director of St. Leo’s, and he was briefly banned from the grounds, a blow for a family who dedicated so much of their lives to the church. Cena has not been able to find work.

Their eldest daughter spiraled into depression and withdrew from the fall semester of college last year. Because the family has a single income and hefty legal bills, their finances have become precarious.

“Lee has been made to look like a monster,” Janice Cena said. “Through this whole thing, we have been praying for truth and justice . . . that is all we ask.”

The cloud was partially lifted six months later, when Fairfax County prosecutors dropped the aggravated malicious wounding charge filed by police. Commonwealth’s Attorney Raymond F. Morrogh said it “did not fit” the circumstances.

Morrogh said the case is unlike any other he has handled. Prosecutors spent months investigating the medical issues and scouring the nation for similar cases to get a sense of what charges were appropriate. There was no consensus.

He said they brought the case to a grand jury late last year with a variety of possible charges, including one that was higher than aggravated malicious wounding. The grand jury returned an indictment on misdemeanor assault and battery, which carries a maximum sentence of 12 months in jail.

Librado Cena declined to be interviewed but issued a statement calling the incident a tragedy for both families.

“The events of the past 9 months have been very difficult for me and my family,” the statement said. “I know that Mr. O’Brien’s family has suffered tremendously as well and my wife and I are deeply sorry for their loss. We have prayed for Mr. O’Brien and his family and we will continue to pray for them.”

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