A Fairfax, Va., man was found guilty today of misdemeanor assault in a "road rage" confrontation last year that resulted in one man's death.

Librado Cena said William O'Brien, 63, was tailgating him and laying on his horn before Cena followed him into a Best Buy parking lot, where he confronted him.

Cena, 58, told police he only punched O'Brien once in his head, but 10 days later that punch would prove to be fatal.

O'Brien was able to make it into the Best Buy to buy an Internet router after his encounter with Cena. But medical officials responding to a 911 call found O'Brien collapsed on the floor of his home just an hour later.

A medical expert testified that O'Brien was taking a blood thinner called Pradaxa for a heart condition, and it played a major role in preventing his blood from clotting after their fight.

Quan Hoang, a witness to the altercation, testified Tuesday that it was O'Brien who appeared to be the aggressor. Cena had said he only punched O'Brien after being punched, kicked and pushed by him.

Hoang said O'Brien even called out to Cena, asking him, "You want more?" repeatedly after Hoang broke up the two.

Cena's defense attorney Kelly Sprissler coupled Hoang's testimony with the Pradaxa factor to counter the prosecution's argument that Cena aggressively sought a fight with O'Brien, which they said qualified as assault.

"What happened between these men on April 16 was a tragedy," Sprissler said. "But it was not a crime."

Prosecutors were pressing for assault and battery charges, but the jury at Fairfax County Circuit Court unanimously agreed on a misdemeanor assault conviction.

Cena is set to return to court for sentencing on Wednesday morning. He could face up to a year in jail and/or a $2,500 fine.