CENTENNIAL, Colorado (CNN) — Police pressed their search for details Saturday explaining the apparent revenge that Karl Pierson sought when he opened fire in his Colorado school while seeking a faculty member.
Investigators have spoken with his parents and will search their home Saturday, but the person who could best answer the question took his own life Friday inside Arapahoe High School in the Denver suburb of Centennial.
Pierson, 18, wounded a student and then shot himself, authorities said. The faculty member was unharmed, and authorities aren't certain whether the student, a girl, was a planned victim or not.
Authorities have much evidence on what Pierson did and how he did it, Arapahoe County Sheriff Grayson Robinson said.
But a full explanation remained elusive a day after the shooting, which immediately took on greater meaning because it occurred on the eve of the anniversary of the Newtown, Connecticut, school shooting, the second-deadliest in U.S. history.
Several witnesses in the Colorado school saw and heard Pierson looking for a faculty member and discharging his weapon.
The halls of the school must have been crowded at the time, the sheriff said. But Pierson's shots hit only one girl, age 15. She underwent surgery and was in critical condition late Friday.
Her blood splattered on another girl, and police initially believed she had been shot, too. But she and everyone else were unharmed -- at least physically. Some of them were treated for panic attacks.
"I'm very grateful for it," Robinson said.
He credited the quick response time for the fact that Pierson stopped firing on others and turned his weapon on himself. Only five minutes passed from the time deputies received the 911 call that they were standing over the teen's deceased body, he said.
Robinson said earlier that first reports indicated a time of 14 minutes or less.
Who was Pierson?
The big question still on Robinson's mind is: "Why?" He has dedicated a team of investigators to answering it.
"That is their key responsibility -- to find out about him, find out what his background was," he said.
Robinson is confident they will find the right pieces to the puzzle, when they execute search warrants Saturday at Pierson's home and at his father's home.
Pierson was a "nice young man," neighbors told CNN affiliate KUSA. He was an achiever, an athlete, who ran cross country. He had worked on an Eagle Scout project, KUSA reported.
The broadcaster had previous contact with him. It got comments from him a few years ago for a feature news story, and he submitted questions to KUSA for a U.S. Senate debate.
High school senior 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. He was honestly incredibly humble and down to earth. He was a little geeky but in a charming way," he said.
Pierson also routinely won contests on his speech and debate team, KUSA reported. He showed off his first place and second place trophies online.
That's where the trouble may have brewed. When Pierson entered the school, he was looking for his debate coach, KUSA reported, sourcing Sheriff Robinson.
Pierson apparently had a "confrontation or disagreement" with a faculty member, Robinson said.
Sandy Hook, Columbine
It was on the eve of the one year anniversary of the Sandy Hook Elementary School massacre that Pierson walked into Arapahoe High School with its student body of 2,229 who occupy 70 classrooms. He was openly brandishing his weapon.
It was only 10 miles away from the site of the Columbine High School shooting, in which Eric Harris and Dylan Klebold killed 12 students and one teacher in April 1999 before killing themselves.
Pierson asked other students about the whereabouts of the man he was looking for, Robinsin said.
A school janitor spotted him in his tactical gear, he told CNN affiliate KMGH.
"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 sthe school.'"
Llerenas said he called 911, and he then escorted the targeted faculty member out of the school.
Pierson had fired at the man but missed, Llerenas told KUSA.
"He was so [shaken] up, he felt the wind hit, out of the shotgun just blew his hair out but it didn't hit him. It was that scary for him."
"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's body was found later in a classroom, Robinson told reporters. The student appears to have acted alone, he said.
In addition to the shotgun, authorities found two Molotov cocktails inside the school, Robinson said.
One of the Molotov cocktails was burning when officers ran into the school, he said. "The deputies encountered a large amount of smoke."
The other was "rendered safe," Robinson said.
Mayhem, lock down
While authorities hunted for the student gunman inside the school, students were locked in their classrooms.
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 and that her teacher told all the students to get into a closet to protect them.
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."
Inside the windowless room were five students and two teachers. "We were shaking, we were crying, we were freaking out," Riley said. "I had a girl biting my arm."
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.
Dozens of students could be seen walking away from the school, with their hands in the air. Some stood in lines at what appeared the high school track field, where they appeared to be undergoing police pat-downs.
Students were then taken by bus to a nearby church where they were reunited with their families, Robinson said. They had to leave their cars in the school parking lot and their belongings in the classrooms overnight.
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."
The sheriff felt it not appropriate to bemoan the shooting as a repeat of the Columbine shooting, but he hinted that the response to school shootings has improved since.
He believes that reaction protocols and communication practices put in place are working.