US rocked by latest school gun attack in Colorado as America marks anniversary of Sandy Hook massacre

Colorado police ask why a ‘charming, humble’ teenager became the country’s newest student gunman

One year on: The President and Mrs Obama observe a minute’s silence for the Sandy Hook massacre (Reuters)

Eight miles away from Columbine and on the eve of the first anniversary of the Newtown massacre where Adam Lanza shot 20 young children and six adults at Sandy Hook Elementary School before taking his own life, a teenage gunman again opened fire in a suburban high school.

Police in Colorado were today investigating why the 18-year-old fired shots in the halls of Arapahoe High School in the Denver suburb of Centennial on Friday, critically wounding a fellow student, before apparently turning his shotgun on himself.

Arapahoe High is eight miles from Columbine High, the site in 1999 of a bloody incident involving two students that left 15 people dead, including the gunmen. At around midday on Friday, Karl Pierson entered his school armed with a shotgun and asked for a teacher, whose name has not been released by authorities.

He is said to have made no attempt to hide his gun. Fabian, a school janitor, told local television station KUSA that he saw the gunman before he entered the building: “I was turning the corner when I saw a kid running into the north side of the building,” he said. “He was kind of running side-to-side, military-style; when I saw that, I double-looked to see if it was a gun. It looked like a shotgun. So, right away I got on the radio to alert everyone.”

Students, meanwhile, were quick to inform the staff member being sought by the gunman who, in what police described as wise tactical move, fled the building in a bid to lure Pierson away from the school.

But a 15-year-old girl who crossed Pierson’s path was critically wounded. Initially, a second girl was thought to have sustained injuries on Friday, but in fact she was covered in blood from the attack on the 15-year-old. Soon after wounding the girl, Pierson is said to have killed himself.

“I believe the shooter knew that deputy sheriffs were immediately about to engage him, and I believe that [he] took his life because he knew that he had been found,” Arapahoe County Sheriff Grayson Robinson told reporters. He also revealed that Pierson had detonated an explosive device, reportedly one resembling a Molotov cocktail, inside the school. A second device had been disabled by authorities. There was nothing to indicate a connection between the incident at Arapahoe High and the Newtown anniversary, the sheriff said.

John Hickenlooper, the governor of Colorado, who was instrumental in toughening up the state’s gun laws in the wake of the shooting at Newtown and another at a cinema in the Denver suburb of Aurora (which is about 15 miles north-east of Arapahoe High), lamented the “all-too-familiar sequence, where you have gunshots and parents racing to the school and unspeakable horror in a place of learning”.

Today, investigators sought to establish a motive; without elaborating further, Sheriff Robinson indicated that officers were looking into the possibility that the attack might have been driven by “revenge”.

Accounts from friends and fellow students portrayed Pierson as a bright member of the Arapahoe High community, who was on the debating and athletics teams. Frank Woronoff, a fellow student, told CNN: “He was the last person I would expect to shoot up a high school.”

He added: “He was honestly incredibly humble and down to earth. He was a little geeky, but in a charming way.”