On April 20, 1999, two teenage gunmen entered Columbine High School in Littleton, Colorado, and began a shooting spree.
In 16 minutes, the gunmen had killed 12 students and one teacher and wounded more than 20 others. They later turned their guns on themselves.
At the time, the Columbine shooting was classified as the deadliest school shooting in U.S. history. A detailed report later criticized SWAT officers on scene, saying it took them too long to reach the wounded.
The scene was chaos. Paramedics were treating victims and transporting them to area hospitals. Fire trucks were parked nearby, acting as a shield for responders to inch in closer to the school and provide medical care.
For Capt. Chris Wells, who was on duty during the incident, the frustration, fear and feeling of not being in control was a foreign concept and inconceivable thought.