Arapahoe High School gunman entered through unlocked door

Updated Dec. 30, 2013 3:55 PM EST

CENTENNIAL, Colo. - The teen gunman who fatally shot a fellow student at Arapahoe High School in surburban Denver earlier this month entered through a door that was supposed to be locked but had been propped open, Arapahoe County Sheriff Grayson Robinson said Monday.

Even if the door had been locked, it probably wouldn’t have deterred 18-year-old Karl Pierson, Robinson said. “He was bent on evil and he was bent on causing the maximum harm to the maximum number of people possible,” he said.

Pierson was carrying a shotgun, more than 125 shotgun shells, a machete and three homemade firebombs, investigators say. He shot and killed himself as sheriff’s Deputy James Englert closed in on him, Robinson said.

The door was one of the few new details released Monday at a news conference on the investigation into the Dec. 13 assault on Arapahoe County High School in the southern suburb of Centennial.

Claire Davis, a 17-year-old senior, was shot point-blank in the face and died Dec. 21. She had been hospitalized in critical condition. Davis’ funeral took place Saturday, and Englert served as a pallbearer at the family’s request, Robinson said.

Englert is assigned as a resource officer at Arapahoe County High, and Robinson credits him with averting more bloodshed by rushing to confront Pierson. “James Englert is a hero, there’s no question,” Robinson said. “James responded heroically and he saved lives.”

An unarmed school security officer, retired sheriff’s deputy Rod Mauler, also ran toward the hallway and room where shots were being fired, Robinson said.

The sheriff said he could not predict when the investigation would be complete.

Investigators say Pierson purchased the shotgun legally a week before the shooting, and they believe he acted alone, but they are still looking into whether anyone else knew of Pierson’s plans. He said Pierson had gone bowling alone on the morning of the shooting.

After the 1999 shootings at Columbine High School, about 8 miles west of Arapahoe County High School, some news reports said the killers had gone bowling that morning, but authorities say that was incorrect.

Twelve students and a teacher were killed at Columbine, and the killers – who were also Columbine students – took their own lives.