Colin Gray, 55, was found guilty on Tuesday of 27 charges in a rare case in which a parent was prosecuted following the actions of a child in a mass shooting.
Jurors took less than two hours to convict Gray after an 11-day trial over his criminal responsibility for the September 2024 deaths of two students and two teachers at Apalachee High School in the city of Winder, about an hour's drive from Atlanta.
Prosecutors told jurors that the defendant armed and enabled Colt Gray, who was 14 at the time of the shooting, including by giving the troubled boy a rifle as a Christmas gift, some eight-and-a half months before the shooting.
His son faces a separate trial on 55 counts including murder and has pleaded not guilty, though the fact that he was the shooter was not in dispute.
Colin Gray held his head low and showed little emotion at the defence table when the verdict was read, as jurors convicted him on charges including second-degree murder, involuntary manslaughter and reckless conduct.
He was later led from the courtroom in shackles, facing the possibility of more than 100 years in prison when he is sentenced.
Piedmont Circuit Superior Court Chief Judge Nicholas Primm dismissed two charges of cruelty to children in the second-degree before the jury was handed the case late on Monday.
Colin Gray had pleaded not guilty and testified in his own defence, saying he was trying to be a good father in a broken home. He said he never imagined his son would carry out any such attack.
Prosecutors said the case was about who armed Colt Gray and who enabled him to take the actions that he did.
The defence argued that the only one who should be punished is the son.
Those killed were students Christian Angulo and Mason Schermerhorn, both 14, and teachers Cristina Irimie, 53, and Richard Aspinwall, 39.
US trials seeking to hold parents criminally responsible for the actions of their children are rare, but have been pursued by some prosecutors in certain circumstances including school shootings.
During Colin Gray's trial in Winder, prosecutors said he ignored warning signs concerning his son.
They said Colt Gray maintained a shrine to school shooters, had behaviour problems at school, vandalised school property with images of swastikas and acted violently at home, including toward his father.
Jurors were shown body-camera footage from law enforcement deputies who came to his home after the shooting.
Colin Gray was seen on the video saying, "God. I knew it, man. My little girl just texted me," referring to his daughter.
The video also showed Colin Gray telling deputies, "We're trying to get him into counselling".
Colin Gray's estranged wife, Marcee Gray, told jurors that her husband ignored efforts to get their son into counselling and that she implored him to keep the guns in the house away from Colt.
On the witness stand, Colin Gray was emotional, testifying, "I could have done more."
