Students returned to classes at Arapahoe High School on Tuesday.
CENTENNIAL - The parking lot surrounding Arapahoe High School reopened to students and staff Saturday morning so they could retrieve their cars left during yesterday's mass evacuation, but investigators will be at the school all weekend, searching for additional clues as to why an 18-year-old senior opened fire.
On Friday afternoon, Karl Pierson entered the school armed with a shotgun, looking for the librarian, the CALL7 Investigators reported. When he couldn't find that staffer he was seeking "revenge"
on, he shot a 17-year-old girl before he killed himself, the Arapahoe County sheriff said.
The girl remains in critical condition at a Littleton Adventist Hospital. Authorities had said she is 15 years old but her friends told 7NEWS that the victim is a 17-year-old senior.
The librarian, Tracy Murphy, was able to get out of the school safely. He learned that he was being targeted, and quickly left "in an effort to try to encourage the shooter to also leave the school," Arapahoe County Sheriff Grayson Robinson said. "That was a very wise tactical decision."
The CALL7 Investigators reported that Pierson was angry at Murphy for kicking Pierson off the school's speech and debate team.
Late Friday night, the bomb squad as well as local, state and federal investigators searched Pierson's Highlands Ranch home. They've also searched his car and his father's Denver home.
The bomb squad also retrieved two Molotov cocktails at the school -- one that had been set off just before the shooting, and one that was never detonated, Robinson said.
Authorities initially said two students were shot, but later clarified that there was only one gunshot victim. The second girl believed to have been shot was covered in blood from the first victim, Robinson said.
The Arapahoe community continues to rally around the victim, who is still fighting for her life. A sign outside the school urges students to contact her family to provide support, saying, "Warriors take care of one another."
Even though there was only one victim -- the entire community is reeling and in pain.
"That's what makes no sense to me ... Why did he shoot the girl? That's my No. 1 (question). Students get mad at teachers all of the time, I just don't get why he would shoot a girl just doing her own thing in the hallway. Makes no sense to me," said senior Max Minne, who was inside the library when the gunman stormed in.
"She was like, 'What are you doing?' And then he just shot her and she just came screaming down the stairs, 'Help me, help me,' and it was just like one of the safest schools in America. You never think it could happen to you and it did," said student Courtney Legtoldt.
Counselors will be on hand from 9 a.m. to 2 p.m. Saturday at Shepherd of the Hills Church and Powell Middle School for anyone seeking help.
Robinson is planning on providing an update on the shooting at 3 p.m. Saturday. We will carry that press conference live on TV, on our website and on our 7NEWS app.
This is Colorado's fourth school shooting to make national headlines -- following Columbine, Deer Creek and Platte Valley. It came one day before the one-year anniversary of the school shooting at Sandy Hook Elementary in Newtown, Connecticut.
Robinson said Friday that the "active shooter protocol" put in place by all local law enforcement agencies following Columbine saved many lives. It's the protocol where responding officers rush toward gunfire - and step over bodies and bleeding victims if necessary - to stop the gunman.
The deputy working as the school's resource officer called in the shooting at 12:33 p.m. and then immediately went to go find and eliminate the threat.
"From the time that the deputy called out the incident until the time that he discovered the shooter's body was within 5 minutes. The deputy did his job. School security did their jobs. And the additional responding deputy sheriff''s that made up the active shooter response team did their job extraordinarily well. I believe that their quick response and their reaction saved lives in this particular incident," Robinson said. "I believe the shooter knew that deputy sheriff's were immediately about to engage him and I believe that the shooter took his life because he knew that he had been found."
Before Columbine, officials followed a contain-and-wait strategy in which arriving officers set up a perimeter to contain the situation, then wait for SWAT team members trained in military tactics to bring down the gunman.
"This kid, the officers went right to him literally within minutes," said Gov. John Hickenlooper. "That is a world of change from the way response used to happen."
Authorities evacuated thousands of students in an orderly procession -making them hold their hands in the air and wait to be patted down before they left school grounds.
It was a demonstration of aggressive security measures developed by police and schools following the 1999 shooting at Columbine, some 8 miles west of Arapahoe High. All of the students there started school after Columbine and grew up in a security-conscious era.
Robinson also said he doesn't believe any additional security measures at the school would have been more effective in preventing the tragedy.
Students heard shots and cowered in classrooms while awaiting word on what had happened. Teachers ushered students in closets and locked doors during the lockdown.
Since Columbine, Colorado has endured other mass shootings, including the killing of 12 people in a movie theater in nearby Aurora in 2012. But it was not until after the Newtown massacre that state lawmakers moved to enact stricter gun control laws.
Two Democratic lawmakers were recalled from office earlier this year for backing the laws, and a third recently resigned to avoid a recall election.
-- The Associated Press contributed to this report.
Coverage: Arapahoe High School Shooting
Teachers and staff went back to work at Arapahoe High School on Monday. Students will return to classes on Tuesday.
An online petition drive is asking Littleton Public Schools to give Arapahoe High School shooting victim Claire Davis a diploma at graduation in May.
When an armed student went into Arapahoe High School on December 13, many people sprung into action.
Claire Davis' father asked people to forgive the fellow student who shot and killed his 17-year-old daughter at Arapahoe High School.
The Arapahoe High School shooter entered the school through a door that should have been closed and locked, but was left ajar, Arapahoe County Sheriff Grayson Robinson said.
Gov. John Hickenlooper orders flags lowered to half-staff Wednesday in honor of Claire Davis, the student slain in the Arapahoe High School shooting.
Tickets to the celebration of Claire Davis' life are now available for students and staff at Arapahoe High School.
Claire Davis will be celebrated next week with a memorial at the National Western Stock Show and Complex.
Prosecutors are warning people that scam artists may try to take advantage of the shooting death of Claire Davis at Arapahoe High School by setting up phony websites and requesting donations.