Growing up, suspect avoided attention.

Why do they do it?

A criminology professor said Friday there are myriad reasons behind mass shootings, such as the one Friday at an elementary school in Connecticut that left at least 28 people dead, including 20 children — one of the worst in U.S. history.

Raymond Corrado at Simon Fraser University said some mass shooters are driven by ideological or political reasons. Others could be suffering from serious mental health issues, delusions or suicidal thoughts or bent on revenge of some kind. Others still could be straight-out psychopaths experiencing a psychotic episode.

Some individuals can be driven by more than one of these factors, he said. Take, for instance, Anders Behring Breivik, who killed 77 people in Norway in a bombing attack and shooting at a youth camp in 2011. Breivik’s antiimmigrant, anti-Muslim rants suggested an ideological motivation, but he also displayed “delusions of grandeur,” Corrado said.

“It’ll take time before you get a history of the individual. It’s very important people realize there are different patterns and different reasons for mass killings,” he said.

“You have to be careful not to lump them all together.”

There is no question, Corrado said, that the American gun culture — where the gun is viewed by many as the “way you solve problems” — is a factor behind why we see so much more gun-related violence in the U.S. compared to other countries.

“You can’t watch a TV program out of the U.S. that isn’t like Pulp Fiction, Desperado, Clint Eastwood. There’s nothing like it. I haven’t seen a culture that is so, in a sense, enamoured with gun violence,” he said.

A list compiled by The Associated Press of the worst mass shootings in the last 50 years showed that 15 out of 25 incidents occurred in the U.S.

Just this year alone, six people were killed at a Sikh temple in Wisconsin in August, and 12 people were killed during opening night of the Batman movie The Dark Knight Rises at a theatre in Aurora, Colo., in July.

Meanwhile, in this country, firearms-related homicides last year reached their lowest point in almost 50 years, Statistics Canada reported earlier this month.