According to Arapahoe County Sheriff Grayson Robinson, Karl Halverson Pierson, 18, entered Arapahoe High School through the west side of the school, which is directly connected to the gym and pool. Robinson confirmed that Pierson was a current senior at Arapahoe High.
Pierson was carrying a shotgun in clear view, with no attempt to conceal it. Robinson said the suspect asked surrounding students for the whereabouts of Tracy Murphy, a librarian at the school.
Murphy learned he was being targeted and immediately left campus. Robinson said Murphy leaving school was an important and wise tactical move, since he was trying to lead Pierson off campus and steer focus away from the students in the school.
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.
After entering the school, Pierson shot a 15-year-old female student, who has since undergone surgery and is now listed in critical condition.
It was initially reported that there was a confrontation between the injured student and the suspect, but Robinson said there was no indication of such an event.
It was also initially reported that a second female student was taken to the hospital with what were thought to be wounds, but it was later determined she only had blood on her from the 15-year-old who was shot, Robinson said.
It was unclear whether Pierson intentionally targeted the 15-year-old girl.
Three other people suffered anxiety attacks: one was transported, and the others were treated and released, he said.
The shooting was first reported at 12:33 p.m. MT. About 20 minutes later, deputies following active shooter protocol discovered Pierson dead in a classroom "in the internal portion" of the school, Robinson said, emphasizing that the suspect made it pretty far into the campus from where he originally entered.
The sheriff said it appeared he turned the gun on himself, though that remains under investigation. Robinson said no law enforcement weapons were discharged on campus.
Two suspected Molotov cocktails were found inside the high school. Authorities said one ignited prior to or during the shooting, causing smoke as police rushed in the high school. A second Molotov cocktail was found on the suspect and was rendered safe.
Officials have since cleared the school and secured the campus. Though the investigation continues, authorities believe Pierson acted alone without any collaborators.
Arapahoe High School will be closed through Sunday, possibly longer. Pierson's body remained in the school as the investigation continued Friday night.
Authorities were investigating the entire school and the area immediately adjacent to it, Pierson's car parked in the school's west parking lot, the home where Pierson was living and Pierson's father's home in Denver.
When the shooting was first reported, all Littleton public schools were placed under lockdown. Streams of students were seen being evacuated from Arapahoe High with their hands on their heads, heading toward the snowy track field.
Students say teachers were ordered to lock classroom doors and dim the lights.
"I was in math and it happened three classrooms away," student Berny Accosta told ABC News. He and other students were assembled at a nearby church following the evacuation.
"There was a first shot and it happened from a classroom, and then I heard three or four more. There was a shot and then a pause and then four continuous shots. We went immediately into lockdown ... turned all the lights off, got as far from the windows and doors as we could," Accosta described. "I saw blood on the ground of the study center when evacuated. That's where there was a lot of blood on the ground. That's where the ambulance carried people out from."
More than 2,100 students attend Arapahoe High, where nine out of 10 graduates go on to college, according to the Littleton Public Schools website.
The school is about 8 miles east of Columbine High School in Littleton, where two teenage shooters killed 12 classmates and a teacher before killing themselves in 1999.
The shooting also came a day before the first anniversary of the school massacre in Newtown, where a gunman killed 20 children and six adults at Sandy Hook Elementary School.
Friday's incident marked the 25th school shooting since Sandy Hook.
The Associated Press and ABC News contributed to this report.
CORRECTION: It was initially reported that two students were injured following the shooting at Arapahoe High School Friday. Arapahoe County Sheriff Grayson Robinson has since said the second student taken to the hospital with what were thought to be wounds was not injured. It was later determined she had blood stains from the other victim, Robinson said.