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Centennial, Colorado (CNN) -- Police are eager to learn why Karl Pierson opened fire with a shotgun at his Colorado school Friday.

But the 18-year-old student lay lifeless overnight on the floor of Arapahoe High School in the Denver suburb of Centennial. His body remained there as part of the crime scene he created when he wounded a student then shot himself, the sheriff said.

Authorities have plenty of evidence to tell them what Pierson did and how he did it, Arapahoe County Sheriff Grayson Robinson said.

Plenty of witnesses saw and heard him looking for a faculty member and discharging his weapon.

The halls of the school must have been crowded at the time, he said. It's a wonder that Pierson's shots hit only one girl, age 15. She was in critical condition late Friday in a hospital, where she was operated on.

Her blood splattered on another girl, and police initially believed she had been shot, too. But she was and everyone else were unharmed -- at least physically. Three 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.

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.

They will work through the night. They have questioned his parents and sealed off the family's suburban home, a gray two-story with solar panels on the roof.

Robinson is confident they will find the right pieces to the puzzle, when they execute search warrants Saturday -- there and at Pierson's father's home.

Pierson was a "nice young man," neighbors told CNN affiliate KUSA. He was a bit of an achiever, an athlete, who showed off his first place and second place trophies online. He had worked on an Eagle Scout project, KUSA reported.

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. "So I don't know anyone who hates him really, so it came as a surprise -- to me at least."

Pierson also routinely won contests on his speech and debate team, KUSA reported.

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. The shooting was an apparent act of revenge.

He did not know if the wounded girl was a planned target or a coincidental victim.

On the eve of Sandy Hook

It was on the eve of the one year anniversary of the Sandy Hook Elementary School massacre that Pierson walked into Arapahoe High School openly brandishing his weapon.

He reportedly wore tactical gear.

It was only 10 miles away from the sight 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 teacher 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 the school.'"

Llerenas said he called 911, and he then escorted the targeted faculty member out of 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'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.

As part of the investigation, authorities also will be looking at school surveillance video, the sheriff 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.

Colton Powers told CNN affiliate KDVR that he heard gunfire during his English class.

At that point, his teacher turned off the lights and locked the door.

"We ran to a corner so they couldn't see us," Powers said.

Within about 14 minutes of the initial report, "we had information on the radio that my deputies believed they had the shooter down inside the school," Robinson said.

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.

The high school, with a student population of 2,229 students, has 70 classrooms. The school, which was built in 1964, is part of the Littleton Public Schools system.

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."

President Barack Obama was briefed on the shooting, according to a White House official.

Ana Cabrera reported from Centennial, and Chelsea J. Carter and Tom Watkins reported and wrote from Atlanta. CNN's Suzanne Presto, Carma Hassan, AnneClaire Stapleton, Mayra Cuevas and Melanie Whitley contributed to this report.