Revisiting one of the deadliest high school shootings in U.S. history.
Police officers led students out of the building, while others stood guard with their weapons pointed at the high school.
An aerial view shows students and police officers crouched behind a car outside Columbine High School.
People ran to safety after they were rescued from the building.
In this image taken from video, a group of freed students and faculty run from Columbine High School.
Dressed in military fatigues and carrying high-powered weapons, members of a SWAT team lead an unidentified man out of Columbine High School.
Emergency personnel wheel a wounded male student at a triage site.
Emergency medical personnel work on a wounded student outside of Columbine High School.
More than 20 people were wounded in the massacre.
The massacre began a little after 11 a.m. when Eric Harris and Dylan Klebold began shooting fellow students outside of the Columbine, Colorado high school.
Emergency responders transported an injured victim to the hospital from the horrific scene at Columbine High School.
Members of a police SWAT march to Columbine High School, as they prepare to do a final search of the school.
This aerial view shows a triage area near Columbine High School.
A woman embraces her daughter after they were reunited following a shooting.
A student is overcome with emotion as he sits down next to a police officer near the triage scene.
(AP PHOTO/DENVER ROCKY MOUNTAIN NEWS/GEORGE KOCHANIEC)
Students listened to a classmate’s experience regarding the shooting during a prayer service on April 21, 1999.
Teenagers mourned the loss of a fellow classmate outside of a parking lot near the Columbine High School.
Members of the community embrace each other a day after the deadly shootings, as they attend a meeting held for families, teachers, students and friends who were affected by the Columbine High School massacre.
During the funeral of 16-year-old Matthew Kechter, mourners stood united with their hands held high at St. Frances Cabrini Catholic Church.
Mourners visit the memorial made up of flowers, cards and balloons to honor the victims of the Columbine High School shooting.