The father of the Arapahoe High School gunman called on school officials Monday to be more forthcoming with details about the interactions they had with his son.

Karl Pierson, 18, walked into the Centennial school Friday afternoon with a shotgun, machete and Molotov cocktails. He shot and seriously wounded 17-year-old Claire Davis, a fellow student who remains in a coma at Littleton Adventist Hospital, before killing himself in the library.

Mark Pierson and his ex-wife, Barbara, released a statement expressing remorse and compassion toward Davis and the school community.

"We are shattered by the tragic events," the parents said. "Our thoughts and prayers are with Claire Davis and her family. They, and she, have suffered unimaginably, and we pray for her full recovery. We also pray for the entire Arapahoe High School community, as we know your lives are forever changed by this horrific event.

"As parents, we loved our son Karl dearly and we are devastated by what happened Friday. We cannot begin to understand why Karl did what he did. We ask for privacy during this unthinkably difficult time and hope that you will respect our need for time to grieve."

Mark Pierson contacted The Denver Post on Monday, wanting to discuss the school's discipline of his son. He said he and his ex-wife, Barbara, attended multiple disciplinary hearings with school officials earlier this year.

He said school officials, including librarian and debate coach Tracy Murphy, should have to answer questions about the interactions they had with his son.

"I'm not the guy who is going to have the answers on this," Mark Pierson told The Post. "Tracy Murphy is the key player."

Murphy could not be reached for comment on Monday. The school district also refused to comment.

Murphy was the target of Karl Pierson as he stormed into Arapahoe High, the sheriff's department has said.

Mark Pierson said his son had not been suspended. He did not want to discuss whether his son issued any threats, and he declined to discuss the case further.

Arapahoe County Sheriff Grayson Robinson said Saturday that Karl Pierson had been nursing a grudge against Murphy and had made a verbal threat against him. One threat made in September resulted in Pierson being disciplined, but he would not elaborate.

Students told The Post that Pierson and Murphy had ongoing confrontations about debate team issues.

One of Pierson's friends said he had told him it was over a threat, that Pierson had said that he wanted to kill Murphy. The boy was not suspended or kicked off the speech and debate team, Robinson said. However, his teammates believed he had been kicked off. And his role as a captain on the team had been removed by October.

Students believed that Pierson and Murphy attended a national debate competition in Alabama in June. But Pierson's father said Murphy did not attend that competition.

An autopsy report released by the coroner's office on Monday confirmed that Karl Pierson died of a self-inflicted shotgun wound to the head.

Pierson was able to purchase the pump-action shotgun legally on Dec. 6 because he turned 18 in September. He bought ammunition for it last week, authorities have said.

The coroner said the cause of Pierson's death was "a severe head injury due to a shotgun wound to the head."

Karl Pierson had wanted to attend the U.S. Air Force Academy in Colorado Springs. Officials say he completed the first step in the application process in June but had not taken any further steps. A formal application was sent to Pierson in August.

Jeremy P. Meyer: 303-954-1367, jpmeyer@denverpost.com or twitter.com/jpmeyerdpost

Denver Post reporter Jordan Steffen contributed to this report