10:30 pm UPDATE: The shooter has been identified as 18 year-old Arapahoe High School student Karl Halverson Pierson.
School and law enforcement authorities learned important lessons from the 1999 Columbine High School shooting in Littleton, Colo., which were evident Friday in another school shooting just a few miles away at Arapahoe High School in Centennial, Colo.
Rather than taking the time to get armed officers in place surrounding the school, those who responded within minutes – police officers and sheriff’s deputies – immediately entered the school to confront and eliminate the threat.
Meanwhile, as soon as shots were heard, teachers immediately followed prescribed protocols, locking doors and ushering students to the back of darkened classrooms.
As described in press briefings Friday by Arapahoe County Sheriff Grayson Robinson, the event unfolded shortly after noon.
A boy identified so far only as a current student entered the school carrying a shotgun, which he made no attempt to conceal.
He asked other students where a particular teacher – also unidentified at this point – could be found. As word spread and 911 calls were being made, that teacher exited the school, intending to draw the threat away from students – a move that Sheriff Robinson praised.
Huddled quietly in their classrooms, some of them weeping, students reported hearing at least three gunshots.
"We were all just sitting there staying quiet and praying," 15-year-old Jessica Girard, who was in math class when she heard three loud bangs, told The Denver Post.
One student – identified only as a girl – was shot and seriously wounded. First reports were that she had confronted the shooter, but Robinson refuted that. Two other students were injured less seriously, although it is not yet clear if that was part of the shooting.
Within minutes, the shooter was found dead in an empty classroom, apparently having shot himself.
As law enforcement officers searched the school for any other possible threats, lines of students could be seen filing slowly away from the school, their hands raised before passing through gantlets of officers conducting pat-down searches.
"The school was evacuated very slowly, very deliberately, and very meticulously," Robinson said. "We wanted to ensure that all of our students were safe."
They were then bused to the Shepherd of the Hills church and the Euclid Middle School to meet their parents.
Friday’s school shooting in Centennial was yet another episode of public gun violence in Colorado – an "all-too-familiar sequence, where you have gunshots and parents racing to the school, and unspeakable horror in a place of learning,” as Governor John Hickenlooper put it at Friday’s briefing.
Arapahoe High School is just eight miles from Columbine High School, where 12 students and a teacher were killed by two students who then took their own lives. And it's 16 miles from a movie theater in Aurora, where 12 people were killed and 58 more wounded in July 2012 during the midnight screening of a new Batman film.
Victims’ advocates and mental health professionals were immediately made available to students, teachers, and school staff.
What is likely to be a lengthy investigation has begun, including interviews with the shooter’s family and friends, as well as a search of his home, computer, and school locker.
Arapahoe High School has 70 classrooms and a student population of 2,229 students.