Friday's mass shooting at a Colorado movie theater is a tragedy we've seen many times before. It's also leading people to once again ask, what causes someone to kill so many people? One national expert based in the Lehigh Valley said there are concrete warning signs of a potential shooter.

"It's really ridiculous," said Todd Pettebine of Slatington, Lehigh Co.

"Something might snap in their mind or something," said Krissa Muschlitz of Allentown.

Allentown psychologist Dr. Peter Langman authored the 2009 book, Why Kids Kill. He has spent more than a decade researching what motivates mass killers, particular school shooters.

"There's no one type [of shooter]," he said. "They tend to fall into three different categories."

And Langman said, each type of shooter is motivated by a different set of reasons.

First there are "psychopathic" shooters, those who dream of "playing God." He gave Columbine mastermind Eric Harris as an example.

"Someone who's very narcissistic, rejects morality," said Langman. "So his ultimate thrill was having the power of life and death."

According to Langman, "psychotic" shooters are driven by fear and paranoid delusions. For example, Virginia Tech shooter Seung-Hui Cho..

"Someone who is schizophrenic, had very poor social skills, whom everyone could tell was a disturbed young man," said Langman.

Finally, there is the "traumatized" shooter. Langman said, think Jeffrey Wiese, who gunned down nine people on a Native American reservation in 2005.

"His father had committed suicide during a standoff with the tribal police," said Langman. "His mother was alcoholic, physically abusive. Jeffrey was suicidal at an early age."

No matter the type of shooter, is there a way to stop them before it's too late?

"It's terrible, but how do you prevent it?", asked Allentown resident Dave Muschlitz.

According to Langman, you often can tell the difference between someone just having a rough time and someone with the potential to kill.

"Sometimes the signs are very public, because they want the world to know," he said. "It's not just that someone dresses funny or says some strange things, but if they have engaged in attack-related behavior."

Behavior like disturbing online postings or videos -- ones that appear to make specific threats. Also watch out for an unhealthy obsession with weapons.

Langman said, sometimes parents or friends should not approach the person directly with their concerns.

"Depending what you know, you may just need to call the police," he said.

He said, if you don't call police, the problem will likely only get worse.

"The person says, 'No, I was just mad,' or, 'I was just joking,'" said Langman. "They blow it off and it does not get addressed."

Langman also believes there is a legitimate fear of copy-cat attacks.

"Often, in the wake of a school shooting, bomb threats and other threats increase dramatically for several days," he said.

Since the Columbine High shootings 13 years ago, Langman said we've gotten a lot better at identifying potential shooters. However, he added that adult suspects like the one in Colorado typically go to much greater lengths to cover-up the warning signs.