A Riverside County jury found John Richard Demery guilty of second-degree murder in the death of 27 year-old Adam Thomas, according to the Riverside County district attorney's office.
Demery, 42, faces up to 40 years to life in prison. He is expected to be sentenced on March 3.
The fatal shooting occurred in the early hours of Nov. 8, 2014.
Demery was at home and off duty when he heard a vehicle screeching through the neighborhood on Cornell Street in Hemet, the district attorney's office said.
The agent, according to authorities, called police and was instructed to remain inside his home, secure his weapon and wait for officers to arrive.
He was sitting in his garage just before 1 a.m. when, after hearing the vehicle again, he grabbed a flashlight and his handgun and decided to photograph the license plate, according to prosecutors.
Thomas had been staying at a nearby friend's home when Demery walked over and an argument over street racing erupted.
Thomas' brother, Brandon Thomas, told the Press-Enterprise his brother was sleeping when he heard a neighbor complaining about teenagers racing. He went outside to ask about what was happening and was shot, his brother said.
Thomas was struck five times by gunfire and later died at a hospital. Thomas was unarmed, officials said.
After the shooting, Demery told investigators he had taken his handgun with him because "he anticipated a potential confrontation," prosecutors said.
"During an interview with detectives, Demery said that he never saw Thomas with a weapon and that the victim never swung at him, only that his fists were clinched at his sides," according to the district attorney's office.
Demery was placed on unpaid suspension after his arrest and is now being terminated, according to Border Patrol Agent James Nielsen. He had been working as an agent since 2008, with his last assignment at the Border Patrol's Murrieta station.
"We are disappointed with the verdict. However, we do not blame the jury," Demery's attorney Richard Pinckard said. "We believe the jurors were diligent in their efforts but struggled with cumbersome and confusing jury instructions which either misstated the law or simply were not applicable to the facts of this case. Our immediate focus has now shifted toward the sentencing process, but we will be evaluating the merits of all post-conviction remedies."
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