(CNN) -- The shooter who opened fire at Arapahoe High School in Centennial, Colorado, was heavily armed, with a shotgun, ammunition, a machete and three explosives, Sheriff Grayson Robinson told reporters on Saturday. The gunman, identified as Karl Pierson, fired five shots, including one point blank at a teenage girl, he said.
Robinson identified the girl as Claire Davis, a 17-year-old senior.
Pierson killed himself less than 1 minute, 20 seconds after entering the school, the sheriff said. He took his own life in the school's library as he was being confronted by a law enforcement officer, Robinson said.
The weapon used by the shooter was purchased legally, the sheriff said.
The victim in the school shooting in Centennial, Colorado, "probably was simply in the wrong place at the wrong time," Robinson told reporters Saturday.
Robinson said that to his knowledge the shooter and the victim, identified as 17-year-old Claire Davis, did not know each other,
"His intent was evil, and his evil intent was to harm multiple individuals," Robinson said.
Why? Sheriff wants to know reason for Colorado school shooting
Police searched Saturday for an explanation to what led a Colorado teenager to take a shotgun to his suburban Denver school, shoot at a faculty member, critically wound a schoolmate and then take his own life.
Investigators have spoken with the parents of the shooter, Karl Pierson, 18, and were planning to search their homes Saturday.
Pierson's shot missed the faculty member; authorities weren't certain whether he had planned to shoot the schoolmate, a 15-year-old girl.
Authorities have amassed a mound of evidence showing what Pierson did and how he did it, Arapahoe County Sheriff Grayson Robinson told reporters on Saturday.
But they were still searching for a full explanation for the shooting, which occurred on the eve of the anniversary of the school shooting in Newtown, Connecticut, the second-deadliest in U.S. history.
Robinson credited a quick response by authorities for the fact that Pierson stopped firing on others and turned his weapon on himself. Only five minutes passed from the initial report of the shooting until Pierson's body was found, dead from an apparently self-inflicted gunshot wound, inside a classroom, he said.
"I believe that that shooter took his life because he knew that he had been found," Robinson said late Friday.
Who was Pierson?
Investigators were trying to determine on Saturday what caused the teen to snap. "That is their key responsibility -- to find out about him, find out what his background was," said Robinson, who added that search warrants were to be executed Saturday at the home where Pierson lived with his mother, and at his father's home.
The 18-year-old was a "nice young man," a neighbor told CNN affiliate KUSA. He was an achiever, an athlete who ran cross country, and he had worked on an Eagle Scout project.
Frank Woronoff told CNN he had known Pierson since they were freshmen together.
"He was the last person I would expect to shoot up a high school," the high school senior said. "He was honestly incredibly humble and down to earth. He was a little geeky, but in a charming way."
Pierson routinely won contests on his speech and debate team and displayed online his first- and second-place trophies, KUSA reported.
But Pierson apparently had had a "confrontation or disagreement" with a faculty member, Robinson said.
Sandy Hook, Columbine
It was shortly after 12:30 p.m., on the eve of the anniversary of the Sandy Hook Elementary School massacre, when Pierson walked into Arapahoe High School, whose student body of more than 2,000 occupy a campus that comprises 70 classrooms. He was making no attempt to hide the shotgun he was carrying.
Pierson asked other students about the whereabouts of the man he was looking for, Robinson said.
A school janitor told CNN affiliate KMGH that the student was wearing tactical gear.
"It just looked weird," Fabian Llerenas said. "He went in and I heard two pops. That's when I knew. I said, 'They are shooting in the school.'"
Llerenas said he called security and then escorted the faculty member out of the school.
Pierson had fired at the man but missed, Llerenas told KUSA.
"He was so shooken up, he felt the wind out of the shotgun just blow his hair out, but it didn't hit him."
Robinson credited the faculty member for leaving the school. "In my opinion, that was the most important tactical decision that could have been made," Robinson said. The faculty member "left that school in an effort to try to encourage the shooter to also leave the school."
Pierson appears to have acted alone, Robinson added.
In addition to finding the shotgun, Robinson said authorities found two Molotov cocktails inside the school.
One of them was burning when officers entered the school; the other was "rendered safe," said Robinson.
While authorities hunted for the gunman inside the school, students locked themselves in their classrooms, as they had been taught to do in drills.
Courtney Leytoldt told KMGH she saw a girl, covered in blood, running down the stairs yelling, "Help me, help me, there's a shooter."
Leytoldt said she was in yoga class when the incident erupted, and that her teacher told the students to get into a closet.
Ninth-grader Whitney Riley was getting ready to grab her computer from her locker when the shooting began.
"We were having fun and laughing and then, all of a sudden, we heard a really loud bang," the 15-year-old told CNN.
"My teacher asked what it was and then we heard two more and we all just got up and screamed and ran into a sprinkler-system room."
Five students and two teachers crowded inside the windowless room. "We were shaking, we were crying, we were freaking out," Riley said. "I had a girl biting my arm."
Through the walls, they soon heard people yelling, and walkie-talkies crackling, and then they heard police asking someone to drop the gun, put the gun down, and hold his arms up, she said.
She did not hear another gunshot, though the people doing the talking could have moved farther away, she said. Soon, they heard police ordering them out.
Video from TV crews who by now had descended on the campus en masse showed dozens of students -- their hands in the air -- walking out of the school. Some stood in lines at a field, where they were patted down by police.
Students were then taken by bus to a nearby church, where they were reunited with their families, Robinson said.
Gov. John Hickenlooper called the shooting an "all-too-familiar sequence, where you have gunshots and parents racing to the school and unspeakable horror in a place of learning."