Meanwhile, Harris and Klebold armed themselves by their cars and waited for the bombs to explode. When the cafeteria bombs failed, Harris and Klebold armed themselves with their weapons, met, and walked toward the building. They went to the top of the West Entrance steps, the highest point on campus. From this vantage point, the cafeteria's side entrance was at the bottom of the staircase, the school's main West Entrance was to their left, and the athletic fields were to their right.
At 11:19 a.m., a witness heard Eric Harris yell, "Go! Go!" At that moment, the gunmen pulled their guns and Harris began shooting at Rachel Scott and Richard Castaldo with his 9mm semi-automatic carbine rifle., who were sitting on a grassy knoll to their left, eating lunch. Scott was hit four times and killed instantly. Castaldo, hit eight times, was critically wounded. It is unclear who shot first and who killed Scott. Subsequently, many rumors have swirled regarding the causes of the rampage, suggesting the possible targeting of Christians. One such rumor included the shooters asking Scott if she believed in God, and then killing her after she answered that she did. The FBI later concluded that this conversation did not take place.
Harris removed his trench coat and took out his 9mm rifle again, aiming it down the West Staircase. Daniel Rohrbough and two friends, Sean Graves and Lance Kirklin, were walking up the staircase directly below the shooters. Kirklin reported seeing them at the top when suddenly they began shooting at him. All three fell, wounded.
Harris and Klebold then turned and began shooting south, away from the school, at students sitting on the grassy knoll adjacent to the steps, opposite the West Entrance of the school. Michael Johnson was hit, but kept running and escaped. Mark Taylor fell to the ground, crippled, and played dead. The other three escaped uninjured. As the shooting continued, Sean Graves stood up and limped down the staircase to the cafeteria's side entrance, where he collapsed in front of the door. Klebold walked down the steps, heading toward the cafeteria. As he descended, he shot Lance Kirklin once more in the face, critically wounding him. As Daniel Rohrbough struggled down the steps towards the bottom of the staircase, Klebold walked up to him and shot him in the back at close range, killing him. He then continued down the staircase and entered the cafeteria, walking over the injured Sean Graves, who lay at the cafeteria entrance. It is speculated that Klebold did this because he was checking to see why the propane bombs had failed to explode. As Klebold stepped into the cafeteria, Harris began to shoot down the steps at several students who’d been sitting near the cafeteria's entrance, wounding Anne-Marie Hochhalter as she attempted to flee. After a few seconds, Klebold returned up the staircase to meet with Harris at the top.
The two then shot toward students standing near the soccer field a few yards away, but did not hit anyone. They threw pipe bombs as they made their way towards the West Entrance, none of which detonated. Inside the campus, teacher Patti Nielson, seeing the commotion, walked towards the West Entrance with student Brian Anderson. She wanted to walk outside and tell the two students to "Knock it off," as she thought they were shooting a video or pulling a prank. As Anderson opened the first set of double doors, Harris and Klebold shot out the windows. Anderson was injured by flying glass and Nielson was hit in the shoulder by shrapnel. Reacting in fear, she quickly stood up and ran down the hall into the library, where she alerted the students inside, demanding they duck beneath desks and remain silent. She then dialed 911 and concealed herself beneath the library's administrative counter. Brian Anderson remained behind, caught between the exterior and interior doors.
Soon thereafter, at approximately 11:24 a.m., a Jefferson County deputy sheriff arrived at the scene and began shooting at Harris and Klebold, distracting them from the injured Brian Anderson. Anderson staggered out of the area and made it into the library where he ran into an open staff break room. He remained there until the ordeal ended. Harris fired ten shots at the officer, who then radioed in a Code 33 (officer in need of emergency assistance). When his gun ran out of ammunition, Harris ran inside the school with Klebold. The pair then proceeded down the main North Hallway, shooting at anyone they saw and throwing pipe bombs. While doing so, they shot student Stephanie Munson in the ankle. She was able to walk out of the school and made it to a house across the street.
Moments earlier, Coach Dave Sanders had evacuated the cafeteria through a staircase leading up to the second floor. The staircase was around the corner from the Library Hallway in the main South Hallway. He and a student turned the corner and were walking down the Library Hallway when they saw the shooters coming around the corner from the North Hallway. The two quickly turned around and ran the other way. The shooters came around the corner and Harris shot at both of them, hitting Dave Sanders in the chest as he reached the South Hallway, but he missed the student. The student ran into a science classroom and alerted the teacher inside.
Meanwhile, the shooters returned up the North Hallway. Coach Sanders struggled over to the science classroom where thirty students had been taking an exam, and the teacher took him in. A sign was placed in the exterior window that read, "I bleeding to death," in order to alert police of their location. Two students administered first aid, and attempted to control the bleeding with the shirts of the male students in the room. A teacher and several students remained in contact with police outside of the school using a phone in the classroom. All the students in the science room were evacuated safely, but Sanders was not evacuated, and died at approximately 3:00 p.m. He was the only teacher killed in the ordeal.
As the shooting unfolded, Patti Nielson was on the phone with emergency services, recounting her experience, and trying to get students to take cover under desks. According to transcripts, her call was received by the 911 operator at 11:25 a.m. The time between the call being answered and the shooters entering the library was four minutes and ten seconds. Before entering, the shooters threw two bombs into the cafeteria from the staircase in the South Hallway, both of which exploded. One can be seen on the security tapes. They then threw another into the Library Hallway, which also exploded, luckily only damaging some lockers.
At 11:29 a.m., Harris and Klebold entered the library where fifty-two students, two teachers, and two librarians were hiding. As he entered, Harris shot at a display case at the opposite end of the administrative counter, injuring student Evan Todd, who was hiding under a copier table nearby. Harris then yelled for everyone to "Get up!" so loudly that he can be heard on the 911 recording at 11:29:18. Staff and students hiding in the library exterior rooms said they heard the gunmen say things like "Everyone with white hats, stand up! This is for all the shit you've given us for the past four years!" and "All jocks stand up! We'll get the guys in white hats!" Wearing a white baseball cap at Columbine was a tradition amongst sports team members.
When no one stood up, Harris was heard saying, "Fine, I'll start shooting anyway!" He fired his shotgun at a desk, not knowing Evan Todd was under it. Todd was hit by wood splinters, but not seriously hurt. The shooters made their way to the opposite side of the library, to the two rows of computers. Evan Todd used the time to conceal himself behind the administrative counter. Kyle Velasquez was sitting at the north end of the computers. Police said that he had not hidden under the desk, but that he was curled up under the computer table. Klebold shot at him first, hitting him in the head and back, killing him. The shooters set down their duffel bags filled with ammunition at the row of computers, and reloaded their weapons.