President Obama and his allies like to point out that all crimes involving guns have guns in common. But they also prefer to neglect the fact that the vast majority of mass shootings have been committed by people with mental illness.

Tuesday’s example comes courtesy of College Park, Maryland, where a student murdered one of his roommates, shot another, then killed himself. According to police, the shooter, Dayvon Green, had diagnosable mental illness. Julie Parker of the Prince George’s County Police explained, “This suspect had had the condition for at least a year, and in the past, he had been prescribed a medication for this mental illness.”