Once the shooting starts, the clock is ticking.
"The number of casualties depends upon two things, one; how quickly the suspect can find victims and two; how quickly we can get there to stop them," said Terry Nichols, with Advanced Law Enforcement Rapid Response Training, or ALERRT, also says active shooter situations in the U.S. are happening more often.
"Just since July alone, I think we're are at a much higher trajectory of the number of events we've had in a number of years," Nichols said.
Each major event teaches law enforcement something new.
"Columbine taught us we need to get there fast and we need to get into the building fast and stop it," Nichols said. "If you remember, the suspect at Virginia Tech chained the doors shut, and luckily it was SWAT teams that were there first...so they were capable of getting in, but normal patrol officers would not have that capability, so now we're training patrol officers in breaching techniques."
"Fort Hood is an example of exterior, where not all shootings happen inside a building," he added.
The plan has always been to stop the violence and set security, but another training shift is coming because of this year's increase in shootings.
Nichols said, "What's realistic is to give them some of the medical skills we've learned the combat over in Iraq and Afghanistan...of massive hemorrhage control. How can we stop and save someone's life that are bleeding to death before EMS gets there?"
Soon, he hopes to also see civilians trained to better handle mass shooting situations where the gunman is looking for a high body count.
Nichols said, "People have their cameras out instead of taking action...that's a new cultural shift...If you hear what sounds like a gunshot go off, already have a plan in your mind, what do I do, where do I go?"
In a perfect world, this training wouldn't be necessary.
"To actually have to train officers to go into a situation like what happened yesterday is unfortunate, but that's the business we're in," said Nichols.
They train and hope everyone learns from shootings, so more of us escape with our lives.
These kinds of shootings tend to happen in clusters. Luckily, more than 40,000 police officers nationwide have received this kind of scenario based training in the past year and a half.