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