Another student also received minor injuries in the shooting.
According to Robinson, the first deputy sheriffs on the scene entered the school immediately and were prepared to engage the shooter. But the suspect was quickly found dead.
"That individual is currently deceased and he apparently killed himself," Robinson said. "That will still be part of the investigation."
The sheriff said the shooter entered the school with a shotgun, looking for a specific teacher. After learning about being called out by name, the teacher exited the school, according to the sheriff. The teacher's decision to exit the building was "the most important tactical decision that could have been made," Robinson said.
"He knew he was the target and he left that school to encourage the shooter to also leave the school with the focus of safety and security and well-being of our students in his mind," Robinson said of the teacher.
He said the students were safer inside their locked classrooms than they would have been had they attempted to exit the school.
Robinson said that he believed there was one explosive device, possibly a molotov cocktail, in the school. A bomb squad was still trying to identify the device.
Robinson said he knows the name of the shooter, who was a student at Arapahoe High School, but the sheriff did not release the young man's name. He said there was "absolutely no indication" that the shooting was tied to the Sandy Hook massacre a year ago.
This post has been updated.