A 14-year-old boy and his father were indicted by a grand jury on Thursday on murder charges connected to the deadly shooting at a Georgia high school last month. The boy is accused of carrying out the attack, and his father is accused of giving his son the rifle used in the deadliest school shooting in Georgia history.

The indictments from the grand jury contain dozens of other charges stemming from the Sept. 4 shooting at Apalachee High School in Winder, Ga., where the authorities said that the boy, Colt Gray, killed two students and two teachers. At least nine other people were injured.

Officials said the boy’s father, Colin Gray, who was arrested the day after the shooting, bore some responsibility because he had allowed the teenager access to the AR-15-style weapon used in the attack. It was a Christmas gift to his son, they said.

Colt Gray now faces a total of 55 counts, including malice and felony murder, as well as involuntary manslaughter and cruelty to children, according to the indictment. Colin Gray, 54, was indicted on 29 counts that include second-degree murder, involuntary manslaughter and cruelty to children.

The charges against the elder Mr. Gray are a test of an emerging strategy by some prosecutors to try to hold the parents of children accused of mass shootings criminally responsible. This year, the mother and father of the gunman in a 2021 school shooting in Oxford, Mich., became the first parents to be convicted in such a case. They were sentenced in April to 10 to 15 years in prison for involuntary manslaughter.

Lawyers for the Grays could not immediately be reached for comment on Thursday.

The indictments followed a preliminary hearing on Wednesday, in which investigators revealed new details about how they say Colt Gray had planned and carried out the attack. They also described signs that the teenager had been troubled in the days and weeks before the shooting.

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