Its a both a mental health issue AND a gun control issue.  There are many people out there who have untreated and/or undiagnosed mental illness with little or no access to treatment.  These folks can become dangers to themselves and others without treatment.  Its a gun control issue because of the apparent ability of these folks to obtain assault style weapons with high capacity magazines or other firearms that could include handguns with extended magazines.  These combination of these two factors, lack of mental health treatment and easy access to guns are the root causes of a uniquely American phenomenon, mass shootings of innocents.  I read a study that noted there were 78 mass shootings in the USA between 1983 and 2013, the country with the next highest number was Germany with 7.  Until we are ready to understand we have a problem with both mental health treatment and access to firearms, and adopt appropriate actions, nothing will change.  Mass killings will continue.