Why Were No Charges Brought Against The Adult Gun Owner???

In watching this documentary it was hauntingly clear that so many adults failed both Larry and Brandon in this tragedy, from the parents, biological and adoptive, to teachers and staff at school, and the system in general. But another adult failed these two boys probably worse than anyone else and that's the adult who owned the gun Brandon used to shoot Larry, which I believe was Brandon's paternal grandfather. It was shown in the documentary that there were multiple guns in the house Brandon lived in, none of which were locked up or even put away securely, and there was even a plastic bag with different types of ammunition laying on a shelf in the very room that Brandon was living in at his grandfather's house. No matter what anyone's opinion is on whether Brandon should or should not have been tried as an adult, the fact is he was 14 years old and kids that are 14 years old can't legally buy guns, and if the guns were securely locked away where there was no way Brandon could access them then he would not have had a gun to take to school that day to shoot Larry with! I'm sure some could try to speculate that he could have possibly bought a gun illegally or harmed or killed Larry by some other means, but it can just as easily be speculated that no one would have died that day if Brandon didn't have such easy access to guns and ammunition in his home. So where's the culpability on the part of the gun owner, and why were no charges filed against him???