Nikolas Cruz is just the latest U.S. mass shooter to legally obtain and own guns because of limited firearms laws, lapses in the background check process or law enforcement's failure to heed warnings of concerning behavior.

The 19-year-old had made threats and was expelled from school, bragged about killing animals, posed with guns on social media and went to a clinic for mental health treatment before took an AR-15-style rifle he had bought legally to a Florida high school and killed 17 people.

The same style weapon has been used in Sutherland Springs, Texas; Las Vegas; San Bernardino, Califo...