A grand jury in Georgia has indicted both a father and son on murder charges in connection with a mass shooting at Apalachee High School in Winder, Georgia. The incident, which took place last month, resulted in the deaths of two students and two teachers, with nine others injured.

14-year-old Colt Gray was indicted on a total of 55 counts, including four counts of malice murder, four counts of felony murder, aggravated assault, and cruelty to children. His father, Colin Gray, faces 29 counts, including second-degree murder, involuntary manslaughter, and reckless conduct. Both are scheduled to appear for arraignment on November 21, where they will formally enter a plea. Colin Gray is currently being held in the Barrow County jail, while Colt Gray, charged as an adult, is being held in a juvenile detention center in Gainesville. Neither has sought to be released on bail.

According to investigators, Colt Gray carried a semiautomatic assault-style rifle on the school bus that morning, with the barrel sticking out of his book bag, wrapped up in a poster board. He then left his second-period class and emerged from a bathroom with the rifle before shooting people in a classroom and hallways. The victims of the shooting were teachers Richard Aspinwall, 39, and Cristina Irimie, 53, and students Mason Schermerhorn and Christian Angulo, both 14. Another teacher and eight more students were wounded, seven of them hit by gunfire.

CNN reports that investigators have said the teenager carefully plotted the shooting at the 1,900-student high school northeast of Atlanta. A Georgia Bureau of Investigation agent testified that the boy left a notebook in his classroom with step-by-step handwritten instructions to prepare for the shooting. It included a diagram of his second-period classroom and his estimate that he could kill as many as 26 people and wound as many as 13 others.

Colin Gray's indictment is the latest example of prosecutors holding parents responsible for their children’s actions in school shootings. District Attorney Brad Smith told the judge at the preliminary hearing, “In this case, your honor, he had primary custody of Colt. He had knowledge of Colt’s obsessions with school shooters. He had knowledge of Colt’s deteriorating mental state. And he provided the firearms and the ammunition that Colt used in this.”

Dan Caplis