Family said Colorado suspect had 'mental illness.' Experts say that's rarely the cause of mass shootings.

After 10 people at a Colorado supermarket were gunned down this week, a question familiar in America's cycle of bloodshed began to echo: Does mental illness drive mass shootings?

The 21-year-old suspect arrested in the rampage, the second in a week, was almost immediately described by family members as paranoid and antisocial.

But researchers and advocates say the rush to cast blame on a mental illness is misplaced.

"There's no psychotic illness whose symptom is shooting other people," said Dr. Jonathan Metzl, director of the Center for Medicine, Health and Society at Vanderbilt University.

"People are searching for explanations for behavior they don't understand. It's easy to put a label like mental illness on behavior that frankly seems just beyond the pale," said Angela Kimball, national director for advocacy and public policy at the National Alliance on Mental Illness.

Less than a day after the shooting, family members of the suspected gunman told The Daily Beast they believed he had a mental illness.

"We didn’t know what was going on in his head," Ali Aliwi Alissa, the suspect’s 34-year-old brother, told The Daily Beast. He also told the outlet his brother was paranoid and would say people were after him when he was a high school student.

"(It was) not at all a political statement; it’s mental illness," he said.

Police have not yet said what the gunman's motive was, but Metzl said immediate blame on mental illness does not tell the full story of what causes a mass shooting.