The gunman who critically injured a 17-year old student at a Denver area high school last week before turning his gun on himself intended to harm “many people,” according to officials who confirmed that he had legally purchased his shotgun earlier this month.

18-year-old Karl Pierson arrived at Arapahoe High School in the Denver suburb of Centennial around midday on Friday, armed with a pump-action shotgun, a machete and three Molotov cocktails. Officials suspect he harboured a grudge against a staff member who had disciplined him in September. On entering the school, Pierson, who is said to have made no effort to conceal his weapons, sought the location of the staff member - a librarian - who was alerted and fled the building unharmed.

Before apparently turning his gun on himself, Pierson wounded a 17-year old girl, named by authorities as Claire Davis, firing at her at point-blank range after she happened to cross his path. She remains in hospital.

The local sheriff said the gunman’s intention was even more devastating. “His intent was evil,” Arapahoe County Sheriff Grayson Robinson said over the weekend. “And his evil intent was to harm multiple individuals.”

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Sheriff Robinson said the gun used in the attack - which occurred on the eve of the first anniversary of the massacre at Sandy Hook Elementary School last year, and took place some eight miles from Columbine High, the site of a bloody 1999 school shooting - had been bought legally by Pierson on December 6 at a local store. Under Colorado law, shotguns can be sold to 18 year olds, while the minimum age to purchase a handgun is 21. The ammunition was bought on the morning of the shooting, the Sheriff said.