MINNEAPOLIS (WCCO) — We may never know why Adam Lanza went on a shooting rampage that killed 26 people including 20 children inside a Connecticut school.

Police say Lanza shot his mother multiple times in the head before going to the school and gunning down 26, and he committed suicide as first responders closed in.

Dr. Kelly Wilson is the founder of the Minneapolis Center for Forensic Psychology.

She said when shootings like this happen, it’s not a quick decision.

“It’s a person who has fantasized about violence for years, who has been smoldering for years, and then there are a series of stressful incidents that occur, and it triggers them to act on something when they’re at the point where they are so despondent they are willing to die for it,” Wilson said.

Police say they have found no possible motive for the shooting and a law enforcement official has said police have found no letters or diaries left behind that could shed light on it.

Wilson said sometimes there is something like a revenge factor.

“When they are reacting to some person or place or some community, and I think the information we have about this particular individual is that he wanted to make sure he had a high count of deaths,” she said.

Wilson said even if we find out what Lanza’s motives were, it would never alleviate the horror of the tragedy.