The Columbine High School massacre changed everything.

In 1999, the suspects took their time and slowly picked their targets while officers devised a plan outside. But no longer are officers forced to wait for the SWAT team to arrive during an active shooter situation, when, sometimes, only one or two officers are available.

RELATED | Las Vegas shooting: At least 58 killed, 500 others injured

Now, after several mass shootings across the nation, the standard adapted: if there's an active shooter and lives are in danger and there's only a few officers ready, they have to be able to act fast and access the threat. Ultimately, if circumstances allow, they must be able to find the suspect and immobilize them.

For officers to get better at tactics, they go through mock city obstacle courses like the ones at the Georgia Public Safety Training Center in Forsyth. There, they role play and learn how to take on active shooters in various scenarios: gas stations, motels, schools, strip malls, and more. One trainer, Bradley McKinney, said in a real situation, the first thing that goes through an officer's mind is their family.

11Alive's Ron Jones shot video last year as he tagged along with officers with the Davis Police Department in California near Sacramento.

Not only were they going through active shooter training, they had fire fighters follow them to treat victims along the way.

According to the Georgia Public Safety Training Center, this active shooter training has been standardized by the FBI and Department of Justice so that every agency across the country are all on the same page, so if a deputy or school resource officer have to take on an active shooter, they know what to do.

RELATED | Officers paint minute-by-minute picture of Las Vegas shooting