"It was clear that he was armed with a shotgun, no effort to hide or conceal it," Robinson said.
Tait Priser, a witness, told ABC News the suspect was wearing a black face cover and a black hoodie.
As shots rang out, the school was immediately placed on lockdown, with teachers dimming the lights and locking their classroom doors.
"I was in math and it happened three classrooms away," student Berny Acosta said from a nearby church where evacuated students assembled.
"There was a first shot and it happened from a classroom, and then I heard three or four more. There was a shot and then a pause and then four continuous shots. We went immediately into lockdown. ... Turned all the lights off, got as far from the windows and doors as we could," Acosta told ABC News.
"I saw blood on the ground of the study center when evacuated. That's where there was a lot of blood on the ground. That's where the ambulance carried people out from," he said
The school is part of the Littleton school district and is just a few miles from Columbine High School, the scene of one of the country's worst school massacres, in which 15 people, including two teenage gunmen, were killed.
The shooting also took place on the eve of the first anniversary of the Sandy Hook school shooting in Newtown, Conn., in which 27 people -- most of them first graders -- were killed.
In a scene that was eerily reminiscent of the Columbine tragedy, hundreds of students could be seen fleeing the school and then approaching police with their hands up to be searched.
Investigators questioned the gunman's family and acquaintances but said they do not believe there are other suspects.