The jury had to determine whether the shooting that left the young officer paralyzed from the waist down was the direct cause of his death 41 years later.
Assistant District Attorney Edward Cameron said the officer had experienced accidents and illnesses that were caused by the paralysis that Barnes inflicted on him.
Defense attorney Samuel Silver argued that Barclay's paralysis was not to blame for his injuries in several car accidents and falls between the November 1966 shooting and his death in August 2007 of a urinary tract infection.
When the verdict was announced, Barnes put his hand to his head, covered his eyes and wept. Barnes' niece, Diane Barnes, exclaimed "Yes!" As the jury left the room, William Barnes said, "Thank you, all."
After the verdict, District Attorney Seth Williams reiterated prosecutors' contention that Barnes was responsible for the officer's death.
"The bullet may not have immediately killed him, but it definitely took away his life," Williams said in a statement. He praised the work of the prosecutors, saying the case was a difficult case to prosecute "both technically and emotionally."
Silver said it was unclear when his client would be released because he had parole violations at the time of his arrest in 2007.
"My brother's been through enough," said Barnes' brother, Jimmy. "He shouldn't have even been on trial."