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Arapahoe County Sheriff Grayson Robinson speaks at a briefing Saturday, Dec. 14, 2013, at Arapahoe High School in Centennial, Colo., where 18-year-old shooting suspect Karl Halverson Pierson shot a student on Friday, then turned the gun on himself. (AP Photo/Ed Andrieski)

CENTENNIAL, Colo. -- The Arapahoe High School shooter was apparently enraged by a conflict with his debate coach and shot a 17-year-old girl he met by happenstance in the hallway within 80 seconds of entering the building, Sheriff Grayson Robinson said Saturday.

"His intent was evil and his intent was to injure multiple people," Grayson said at a news conference Saturday afternoon. "I believe (victim Claire Davis, 17) was simply in the wrong place at the wrong time."

The quick actions by a sheriff's deputy and security officer undoubtedly saved lives, Robinson said

Shooter Karl Pierson, 18, legally bought a pump-action shotgun Dec. 6 and a large amount of ammunition Friday morning, Robinson said.

Robinson declined to say what evidence was discovered at Pierson's home Friday night and Saturday, but he added that it appeared he acted alone.

"We found nothing in those search warrants that there were any co-conspirators," Robinson said. Pierson had been disciplined by his supervisor, but had not been suspended and remained on the debate team, he said.

Robinson also read a statement from Davis' family, who indicated that first responders and staff at Littleton Adventist Hospital saved their daughter's life. The note thanked the many people who have prayed for their daughter. Her father told 7News Claire Davis was "not doing very well."

They added that she suffered severe head trauma from a gunshot and remains in critical condition. They asked for their privacy.

From the time Pierson entered the school until he apparently shot himself to death, only 80 seconds elapsed, according to videotape reviewed by investigators.

Robinson corrected timelines and facts he said were based on the best information available Friday. Pierson parked in the north parking lot near the library. He was wearing a bandolier loaded with ammunition and carried a backpack with three Molotov cocktails. He was also armed with a machete.

Pierson fired his shotgun after entering a door near the library on the north end of the school rather than the west entrance as first believed. He fired his shotgun five times, ignited a Molotov cocktail and set the library on fire before turning the gun on himself. He fired the shotgun at point blank range in Davis' face.

"There was no time for the victim to run from the shooter," Robinson said.

The deputy, who was assigned to the school, and a security officer immediately dashed from the cafeteria down a long hallway to the library, while yelling for students to get down.

"The shooter knew that the deputy was coming," Robinson said. He added that he had no doubt that his courage saved lives.

Arapahoe County sheriff's investigators are interviewing students, teachers and staff members Saturday in an effort to learn every tiny detail about the shooting.

By all accounts Pierson had not targeted Davis, Robinson said.

"She's very innocent, humble and sweet, no matter what," said Afton Phelps, a senior who is friends with Davis and once went in the same group to a homecoming dance.

Phelps said Davis is a talented equestrian whose bedroom decor revealed her love of horses. The two had been closer in their sophomore year, but occasionally grabbed lunch together.

"It's hard to wrap our minds around," Phelps said of the shooting. She and two of Davis' other friends sought counseling on Saturday and found many of their classmates doing the same. "It was really helpful to talk about it."

Pierson was an 18-year-old senior who had been a member of the school's debate club.

The sheriff said Saturday he is currently discussing details about the shooting including how the librarian and debate team coach, Tracy Murphy, who may have been the target of the shooting, made it out of the school unharmed.

Robinson said that his deputies are annually trained in how to respond to an active shooter scenario at a school or public place.

"It's an obvious focus of what we train for every year," Robinson said.

The high school was still surrounded in a ribbon of yellow crime scene tape as students trickled back to Arapahoe High School Saturday morning to reunite with their cars and with each other.

Among them were Chris Davis and two other seniors who taped a cardboard sign to the fence, urging their classmates to raise money for the wounded student who remained hospitalized.

"I don't know her very well. We're just trying to help her family in any way we can," Davis said. The morning announcements at the school end with a reminder that, "Warriors take care of one another," he said. "Right now we are taking care of a fellow Warrior and her family."

Other students were still shaken and struggled to find words.

"Waking up today was kind of rough," said Emily Morris, a senior who came to retrieve her car. "You realize it's not a nightmare like everyone wanted it to be."

On Saturday, Morris' concerns were for the wounded Davis, with whom she would grab lunch and share schoolwork.

"She is the nicest girl I've ever met, she doesn't deserve this at all," Morris said. "I don't know why anyone would do this to her. It's all so confusing."

But perhaps more confusing is what lies ahead, she said.

"I don't know what it's going to be like to go back inside the school."