Littleton school district officials admit warning signs were missed leading up to the deadly Arapahoe High School shooting in 2013, according to school district reports released Monday. A threat assessment by the district was conducted on the shooter, Karl Pierson, just months before the attack. Yet he was deemed a low risk, and allowed to return to school. Now, the superintendent of Littleton Public Schools admits the threat assessment was incomplete. More: Arapahoe High School Shooting Reports Show Missed Signs About Gunman.