July 30 (Reuters) - A man who gave his troubled son the rifle he used to kill four people at a Georgia high school was sentenced on Thursday to 15 years in prison, ​the culmination of the state's first case in which a parent was convicted of ‌second-degree murder for a mass shooting committed by his child.

Colt Gray, 16, was sentenced to life in prison without parole on Tuesday for the 2024 school shooting by a Barrow County Superior Court judge. Two days later, the same judge ​sentenced his 55-year-old father, Colin Gray, to far less than the 80 years prosecutors had ​requested.

Judge Nicholas Primm noted that none of the charges underlying Colin Gray's conviction ⁠entailed mandatory prison time while acknowledging the "immeasurable heartache" his negligence had caused.

"The loss here is profound," ​Primm said, following tearful testimony by several relatives of those killed in the shooting. "But I still have ​to distinguish your acts from the malignant heart and the acts that Colt Gray committed."

Colin Gray is the latest U.S. parent to be prosecuted for a mass shooting committed by a child -- an emerging legal strategy employed by prosecutors in ​a bid to tackle the epidemic of gun violence in U.S. schools.

Months before Colt Gray's shooting, ​the parents of a Michigan teenager who shot and killed four classmates were sentenced to 10 to 15 years in ‌prison after ⁠being convicted for manslaughter.

In that case, a jury found Jennifer and James Crumbley guilty of not securing guns in their home and of ignoring signs that their son was mentally disturbed.

Colin Gray's defense attorney on Thursday urged the judge to give his client a sentence of 10 years in prison and ​10 years in probation, ​arguing the Michigan parents ⁠had more advance notice of their son's problems and noting that Gray was the first parent in the state to be charged for a child's ​mass shooting.

"We would ask the court not to make Georgia the national ​outlier in this ⁠first case of its kind, against a defendant whose warnings came with less and whose efforts were more," Hobbs said.

Barrow County District Attorney Brad Smith rejected Hobbs's claim that Colin Gray had little warning of ⁠his son's ​violent plans, showing the judge a replica of a bedroom ​wall on which Colt had created a shrine to the gunman behind the 2018 mass shooting at a high school in ​Parkland, Florida.

The risk was "staring him in the face every day," Smith told the judge.