Colorado shooter Karl Pierson had written plans to attack at least five areas of Arapahoe High School on his arm , as well as the Latin phrase for `` The Die Has Been Cast , '' the Arapahoe County Sheriff 's Office said Tuesday . Specifically , he 'd written five letters/ numbers on his arm , corresponding with the library and classrooms near it , that investigators believe indicate what he planned to do . `` Teams of Sheriff 's Office investigators will attempt to determine the relevance of the list of numbers written on the shooter 's arm to specific classrooms , while closely evaluating if the occupants of those classrooms may have also been targeted for harm by the shooter , '' the Sheriff 's Office said in a statement . According to police accounts , Friday 's shooting began at 12:33 p.m. when Pierson , 18 , walked into Arapahoe High School armed with a pump-action shotgun , a machete and a backpack containing three Molotov cocktails , a bandolier of ammunition across his chest . He shot 17-year-old Claire Esther Davis once in the head amid shots fired randomly into school hallways . She is in critical condition . He also ignited one of the firebombs in the school library before killing himself in the back corner of the library . Police say Pierson was looking for revenge against a librarian but could n't find the faculty member and killed himself as a school resource officer closed in . Davis was just a random victim , police surmised , and the gunman and victim did n't know each other . Pierson 's parents released a statement Monday afternoon , offering prayers for Davis ' full recovery . `` As parents , we loved our son Karl dearly and we are devastated by what happened Friday . We can not begin to understand why Karl did what he did , '' Barbara and Mark Pierson said . They also offered prayers `` for the entire Arapahoe High School community , as we know your lives are forever changed by this horrific event . '' Colorado 's school shooting -- over in 80 seconds . In Arapahoe texts , bonds between father and son shine through .
Arapahoe High School had written plans to attack at least five areas of the school on his arm . He had also written the Latin phrase for `` The Die Has Been Cast According to police , Claire Esther Davis shot 17-year-old Karl Pierson once in the head .