The father of a teenage boy who shot and killed four people at a high school in the US state of Georgia has been sentenced to 15 years in prison for murder, manslaughter and other charges.

Colin Gray, 55, was found guilty in March on a slate of charges stemming from the 2024 attack that left two teachers and two students dead in Winder, Georgia.

His son, 16-year-old Colt Gray, pleaded guilty to the murders last week, and investigators said that the rifle he used was a gift from his father.

Colin Gray was "the one person who could have prevented" the shooting from happening, prosecutors said.

The attack at Apalachee High School outside the city of Atlanta claimed the lives of Christian Angulo, 14, Mason Schermerhorn, 14, Richard Aspinwall, 39, and Cristina Irimie, 53.

Colin Gray's prosecution marks the third time a US parent has been held criminally responsible for a mass shooting carried out by their child, according to CBS News, the BBC's US partner.

Judge Nicholas Primm handed down the sentence on Thursday, giving the defendant 15 years for each second degree murder conviction and 10 years for each involuntary manslaughter conviction, all to be served concurrently.

Prosecutors in Georgia had asked for the maximum punishment of 80 years in prison, while the defence asked the judge for a lighter 10-year sentence.

Judge Primm spoke directly to Gray before announcing his sentence, admonishing him for failing to send his son to counselling, as well as failing to monitor his online activity and lock up the guns in the house.

"It is clear you failed as a parent," he said. "The warning lights were flashing brighter and brighter...and you failed to do the simplest things to prevent this."

But Primm also acknowledged that Colin Gray "didn't seek to harm anyone" and "didn't pull a trigger" himself.

During the trial, the jury heard how the father had bought his son an AR-style rifle for Christmas the year before the attack, even though the boy had been questioned by police just seven months earlier about online threats to commit a school shooting.

Prosecutors argued he ignored numerous warning signs, including a notebook detailing how his son planned to kill students and teachers.

"After seeing sign after sign of his son's deteriorating mental state, his violence, his school-shooter obsession, the defendant had sufficient warning that his son was a bomb just waiting to go off," Barrow County Assistant District Attorney Patricia Brooks told jurors.

"And instead of disarming him, he gave him the detonator."

His lawyers attempted to shift blame to his son.

"This is the person who went into the high school and shot and killed four people he didn't even know and injured scores of others," his lawyer, Jimmy Barry, told jurors, referring to the boy.

"This is the person who needs to be punished. He made a conscious decision to do this - a secretive decision."

Charges against the parents of a suspect in a school shooting are relatively new in the US, with at least two other cases being brought to trial recently.

In April 2024, the parents of a Michigan teenager who killed four students with a gun they had bought for him just days before the shooting were sentenced for their role in the attack.

The case was widely reported to be the first time the parents of a child who had carried out a mass shooting in the US were held criminally liable.