Police protocol for active shooters varies across departments.

Ken Crane, President of Phoenix Law Enforcement Association (PLEA), says protocol has changed over the years.

Today at the police academy, Crane says, cadets are taught to enter an active shooter situation with a three-man team.

"Back in the day, it was get a four-man team and go in," Crane said. "Now they're (experts) saying the quicker you can apply pressure."

Crane says even though it's suggested to have a three-man team, that can change.

"That may mean one guy going in," Crane said. "That may mean two guys going in but we're not going to wait for four. The quicker we can go in there, locate, try to put pressure on, the quicker we are going to stop it and distract that guy from his original mission."

Phoenix Police Sergeant Jon Howard says he cannot provide specifics regarding response protocols.

"The expectation that a small group of officers enter the building together, as quickly as possible, sounds appropriate but there are too many unknown factors to make that a hard and fast rule," Howard said. "It would depend on too many of those factors to determine the best response for any given scenario."

Crane says School Resource Officers or "SROs" can play a big role in stopping a school shooter.

"Nobody is going to know that school better than the SRO," Crane said.

"They're usually the best to know what to do and how to navigate that school under stress," Crane said.