A schoolboy who killed a senior teacher calmly accepted a suspension for driving on an athletics field hours before he opened fire with his policeman father's gun.
Robert Butler went home to speak with his father and some friends, giving no hint of the horror to come.
But after his father left to run errands, Butler, 17, took his service handgun from a cupboard and headed back to Millard South High School in Omaha, Nebraska, to confront the administrator who had punished him.
Butler, whose father is a detective for the Omaha Police Department, asked to see assistant principal Vicki Kaspar and shot her in her office. He then wounded principal Curtis Case and fled before killing himself. Ms Kaspar, 58, died in hospital hours later.
Police outlined the events that led up to the shooting, which unfolded at the school on the first day of class after the Christmas break.
Butler had transferred to Omaha in autumn from a school in Lincoln, about 50 miles away. On New Year's Day he had been cited for criminal trespassing after driving his car on the school's football field and track.
Butler was called out of class at 8.10am on Wednesday to see Ms Kaspar. The two talked in her office and he was escorted out of the school at 9.23am. But Butler had remained calm after learning of the suspension.
"He wasn't acting like an out-of-control student at all," police chief Alex Hayes said. The teenager received a ride home, where he talked to his father and spoke to some friends by phone.
Butler's parents decided to transfer him to Omaha because he was having disciplinary problems in Lincoln and had not been listening to his mother, who is divorced from his father, Mr Hayes said. Butler's father had no any reason to expect his son would turn violent because he did not seem distraught about the suspension and had no history of mental illness.
"He was disappointed with the discipline, but he wasn't acting angry," Mr Hayes said.