Littleton, Colorado. April 20, 1999.
Two heavily-armed students go on a shooting rampage at Columbine High School.
The first officers on the scene wait for a SWAT team to arrive.
By the time that happened, Eric Harris and Dylan Klebold had killed 12 students, one teacher and wounded 23 others, before killing themselves.
Law enforcement agencies all over the country learned from the Columbine tragedy.
Lt. James Vestal, of the Bay County Sheriff's office, said, "Some of the victims that actually died of that shooting actually bled out because nobody would enter that building. They waited for the SWAT team, they waited for a specialty unit to respond. Then they went in. By that time, over an hour and a half had expired."
That's why Bay County sheriff's deputies took to the halls of the old Bay County jail Wednesday afternoon, stalking active shooters.
Vestal said, "In the past, all agents have basically waited on a specialty team, SWAT team, CERT team, one of these specialty units. They figured out they don't have time to do that anymore."
The training is extremely realistic.
Each team of four deputies is armed with standard-issue Glocks.
The guns fire nine-millimeter shells filled with paint, called "sim"-unition, so deputies know if they've been hit.
The deputies participated in the simulated stand-offs, learning how to react if they are the first responders.
They went through a number of scenarios, from an ambush in Gulf Coast Medical Center, to a hostage situation at a local church.
Vestal said, "I feel a whole lot better, at least in this agency, that if my children are in a school and somebody's shooting, actually shooting them, that the first four people there, I have no worry they will know what to do to go ahead and take a person and engage them."
This type of training is a big change for law enforcement, giving nearly all deputies the ability to respond to any hostile situation with an armed shooter.