As they investigate the latest school shooting in the United States – Friday at Arapahoe High School in Centennial, Colo. – one thing is clear to law enforcement officials there: The presence of an armed deputy sheriff on regular duty at the school was the key factor in preventing more deaths and injuries.

As soon as he heard the first of five gunshots, that officer and the two school administrators he was talking to raced toward the commotion shouting their presence and ordering students and staff to follow the school’s lock-down protocol.

As a result, Arapahoe County Sheriff Grayson Robinson said at a briefing Saturday afternoon, the heavily-armed shooter realized he was about to be confronted by an armed officer, and he took his own life.

“We believe that that action was absolutely critical to the fact that we didn’t have more deaths and injuries,” Sheriff Robinson said. The whole episode – from the time the shooter entered the school until he shot himself – lasted just one minute and 20 seconds.

In the years following the 1999 massacre at Columbine High School in Littleton, Colo., the public and school officials have debated whether it’s right to turn schools into what some critics call “armed camps.” Arapahoe High School with its 2,229 students and 70 classrooms is an open campus with no metal detectors, although it does have that armed deputy on duty every day classes are in session.

The shooter at Arapahoe High School – 18 year-old student Karl Halverson Pierson – was armed with a pump-action shotgun, two bandoleers filled with shotgun shells, a machete, and a backpack carrying three Molotov cocktail-style bottles of flammable liquid, one of which he ignited. His arsenal had the potential to do significant damage.

With his considerable weaponry, Pierson intended to confront librarian Tracy Murphy, who was also the young man’s debate coach.

Sometime in early September, the two had argued over something. Mr. Murphy had disciplined Pierson (although he did not drop Pierson from the debate team as had been reported). Several other students heard the confrontation, including Pierson’s threat to Murphy. Why the threat to Murphy was not more significantly dealt with is unclear.

On December 6, Pierson legally purchased a pump shotgun. (Eighteen year-olds in Colorado may purchase “long guns” – shotguns and rifles – but not handguns until they are at least 21.) The day before the attack, Pierson bought more rounds of ammunition.

He entered the school at about 12:30 pm Friday, shouting for Murphy. When the librarian heard the commotion, he immediately left the school, accompanied by a janitor. The idea, according to Sheriff Robinson, was to draw the threat away from the school.

Pierson fired several random shots, hitting Claire Esther Davis, a 17 year-old senior, at close range. Following surgery, Ms. Davis is in critical condition at a local hospital. “She needs your continued prayers,” her family said in a statement.

Apparently, the young woman was not an intended target but just “in the wrong place at the wrong time,” as the sheriff put it. No one else was hit by the five rounds Pierson fired, although several students were treated for what officials describe as “anxiety attacks” following their ordeal locked into darkened classrooms then being marched single-file out of the school with their hands up and being frisked while police and sheriff’s deputies searched for any other possible shooter.

Authorities now know that Pierson acted alone. Law enforcement authorities obtained warrants to search the homes of Pierson’s parents (the couple is divorced) as well as his car.

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“We will continue to do interviews and investigate evidence,” Robinson said Saturday. “We will not do it quick; we will do it right.”

In any case, says Robinson, “It is my strong opinion that his evil intent was to harm multiple individuals.”