The 14-year-old boy accused of killing four people inside of his Georgia high school last month allegedly smuggled a rifle into the school by hiding it inside of a poster board, a court heard Wednesday among many disturbing new details about the case.

The teen wrapped the AR-15-style rifle in a poster and carried it into Apalachee High School in his backpack on Sept. 4, a special agent with the Georgia Bureau of Investigations testified during a court hearing for the students father, Colin Gray. The father and son were indicted by a grand jury Thursday on dozens of counts related to the attack.

The rifle was protruding from his bookbag but it was not immediately obvious to agents what it was, GBI Special Agent Lucas Beyer said Wednesday after reviewing security footage from the school and the teens school bus. At a quick glance, it appeared that [he] was possibly transporting a school project.