More than half a dozen others were injured before the police fatally shot the gunman in a third-floor room where he had barricaded himself.
But Missouri is one of 29 states that have no background check requirement for private sales.
So, Mr. Harris found a weapon by browsing the online site Armslist.
Federal law requires background checks only for purchases made through the approximately 80,000 businesses that sell, ship, import or manufacture weapons licensed through the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.
Unlicensed private sellers, by contrast, can legally sell their wares at gun shows, out of their houses and, increasingly, through online platforms such as Armslist that match buyers with sellers.
Persons:
Louis, Orlando Harris, Mr, Harris, Alexzandria Bell, Jean Kuczka
Organizations:
Visual, Performing Arts High School, of Alcohol, Tobacco, Firearms, Explosives
Locations:
St, Missouri