Chester Police Commissioner Joseph Bail has studied mass shootings here and abroad. He co-wrote a book about the 2007 Virginia Tech incident that cost 32 lives.
Bail says arriving police officers confront the unknown like how many shooters and what kind of weapons, so they must be hyper alert to threats.
"The big thing we try to make sure is that the officers don't have tunnel vision. Don't focus just on getting through the door. See what's around the door, who is coming through the door, see who is on the other side," said Bail.
So who are mass shooters? Dale Yeager - a Criminal Behavior Analyst and profiler says most mass shootings are public suicides.
"They were already going to commit suicide now they're going to go and take other people with them," said Yeager.
In the Aurora, Colorado incident that was not the case.
"He did not commit suicide, he gave up very easily. He was on a mission, he completed his mission and he gave up," said Yeager.
Yeager thinks that is significant and that James Holmes may have a cause.
"It reeks that the shooter was motivated by a political or some kind of social ideology," said Yeager.
He also says if you dig deeper into those who profess a political motivation for shooting, you still find that their lives are chaotic.
Yeager says more will be learned as Holmes' past is revealed.