Parents of Colorado high school shooting gunman ‘shattered’ (LIVE VIDEO)

The parents of Karl H. Pierson, who opened fire in a Colorado high school on Friday critically injuring one student before turning the gun on himself, said Monday they were "shattered" by the shooting.

"We are shattered by the tragic events that took place on Friday at Arapahoe High School," Barbara and Mark Pierson said in a statement.

The couple said they were praying for the 17-year-old female student shot by Pierson as he searched for the teacher who had kicked him off the school debate team. 

"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." 

Pierson made no effort to conceal his shotgun when he entered the high school in suburban Denver at 12:30 p.m. local time with the intention of confronting a teacher, police said.

The teacher slipped out of the building in the hope of drawing Pierson away from the other students. He shot Davis in the head before killing himself. 

Davis remains in a critical condition in hospital. A second student initially thought to have been injured in the shooting was in fact covered in blood from Davis.  

Friday's shooting came on the eve of the anniversary of last year's massacre at Sandy Hook Elementary School in Newtown, Conn., in which 26 people, including children and teachers, were shot dead. 

Arapahoe County Sheriff Grayson Robinson said two Molotov cocktails were found at the school. 

Robinson praised the teacher for making "the most important tactical decision that could be made" by leaving the building. 

"He knew he was the target and he left that school in an effort to try to encourage the shooter to also leave the school," Robinson said.

"That was a very wise tactical decision."

One report said Pierson was in a "rage" on Wednesday after the teacher kicked him off the school's debating team.

Police are now talking with Pierson's family to try to work out why the teenager walked into the school and opened fire. 

Arapahoe High School is about 10 miles from Columbine High School, the scene of a 1999 massacre in which two students murdered 12 students, a teacher and wounded several others before the pair committed suicide.