Arapahoe County Sheriff Grayson Robinson said Friday's shooting at Arapahoe High School by 18-year-old Karl Pierson was likely motivated by retaliation against a faculty member, probably a librarian at the school.
Robinson said it looks like the librarian was the initial target, but that the gunman planned to hurt multiple people, evidenced by the large amount of ammunition he brought with him.
The sheriff said Pierson bought the pump-action shotgun legally Dec. 6 at a local retail store. He also said the teen bought ammunition legally the morning of the shooting. Anyone over 18 is allowed to buy a shotgun in Colorado; only those over 21 can legally buy a handgun.
Robinson identified the wounded student as 17-year-old Claire Esther Davis. It was initially reported that the injured student was 15 years old. Robinson said she was likely not a target but was in the wrong place at the wrong time.
"She is an innocent young lady and she was an innocent victim of an evil act of violence," he said.
At a Saturday afternoon news conference, the sheriff read aloud a letter from Davis' family. According to the letter, the teen remains in critical condition with severe head trauma as a result of a gunshot.
Officials initially said Pierson entered the school on the west side. Robinson clarified Saturday that the shooter actually entered from the north side. The sheriff said officials have reviewed surveillance video, which showed that the entire sequence of events took just 1 minute and 20 seconds - from the time the gunman entered the school to when he took his own life.
According to Robinson, after the gunman entered the school, he fired a random round down a hallway. He then shot Davis point blank, then fired another round down a hallway. He then entered the school's library/media center, where he fired another random round and detonated one Molotov cocktail, setting three bookshelves on fire. He then fired a fifth round, ran to the back corner of the library and took his own life.
Sometime within this timeline, officials say Pierson asked surrounding students about the whereabouts of Tracy Murphy, a librarian at the school. Officials say Pierson made no attempt to hide his weapon.
According to ABC News affiliate KMGH-TV in Denver, Murphy was in charge of the debate team and had reportedly kicked Pierson off the team recently. Fellow students describe Pierson as outspoken and smart - not a loner. They also say he held communist views.
Quick-thinking students alerted Murphy, who quickly left school grounds. Pierson critically wounded a 15-year-old student, but the strategic response by police on the eve of the Newtown massacre anniversary appears to have averted more bloodshed.
After the shooting was reported at about 12:30 p.m. MT, authorities evacuated hundreds of students in an orderly procession - 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.
After that tragedy, police across the country developed "active shooter" training in which responding officers rush toward gunfire to stop the gunman.
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.
The Arapahoe High shooting came a day before the anniversary of the Newtown, Conn., attack in which a gunman killed 20 children and six adults at Sandy Hook Elementary School.
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.
The Associated Press contributed to this report.