OMAHA, Nebraska (AP) — The 17-year-old gunman who opened fire at his Nebraska high school, killing an assistant principal, had been suspended from class and removed from the building hours earlier for driving on an athletic field, police said Thursday.

Robert Butler Jr. was escorted out of Millard South High School on Wednesday morning. A few hours later, he returned and shot both the assistant principal and the principal before fleeing and killing himself, police said.

Butler, the son of an Omaha police detective, had transferred in the fall from a school in Lincoln, about 50 miles (80 kilometers) away. He had been cited on New Year’s Day for criminal trespassing after driving his car on the school’s football field and track, police said.

Butler was called out of class Wednesday morning to meet with Assistant Principal Vicki Kaspar. After the two talked in her office, he was escorted out of the school, police said.

WATCH THE AFTERMATH OF THE SHOOTING HERE

Butler walked back into the school’s main entrance several hours later and signed in at the office to meet with Kaspar. Police said he had been in her office about four minutes with the door closed before he shot Kaspar. Butler than shot Principal Curtis Case.

Police said an unarmed security officer was sitting at a desk outside the offices and saw Butler walking away. Butler pointed a gun at the officer, who took cover and wasn’t shot.

Butler also fired at a custodian and missed, and bullet fragments apparently hit a school nurse, who was treated at the scene.

A school resource officer who heard the gunfire called police.

The security officer gave police Butler’s name and described his car, and authorities received a report of a suspicious vehicle about a mile (1.6 kilometers) from the school. Officers raced to the car and found Butler inside, dead from a self-inflicted gunshot wound.

The chaos unfolded on the school’s first day back in session after the holiday break.

Kaspar, 58, died Wednesday evening. Case, 45, was hospitalized in serious but stable condition.

Sometime after he was escorted out of the building, Butler posted a rambling message on Facebook about his unhappiness with his new school, but he didn’t supply many details. Instead, the expletive-laced note predicted Butler’s friends would hear about “evil things” he did.

He wrote that the Omaha school was worse than his previous one, and that the new city had changed him. He apologized and said he wanted people to remember him for who he was before affecting “the lives of the families I ruined.” The post ended with “goodbye.”

A former classmate of Butler’s from Lincoln confirmed the Facebook post to The Associated Press and provided AP with a copy of it.

Copyright 2011 The Associated Press.