While mass killings have been seen throughout American history, serial killing began to be recognized as a separate issue in the 1970s, while mass killings began to be designated separately because of high profile cases in the 1980s.
When Charles Whitman opened fire on a Texas university campus in the 1960s, it became the quintessential example of public, random killings in which being in the wrong place at the wrong time could be deadly.
While some killers commit calculated and planned murders, it's far more likely for a mass murderer to go on an impulsive, rage filled killing spree.
Many mass killings have been committed against people of specific races and religions in an act we define as a hate crime.
It's usually a place of security, but workplaces can become the target of a disgruntled worker with a gun.
While Columbine is one of the most well known examples of a school massacre, the phenomenon is not a new one.
In 1975, on Easter Sunday, James Ruppert decided to kill 11 of his family members. While it was the largest family killing in American history, it would not be the last.
Cult leaders attract devoted and fanatic followers, and sometimes convince them to take their lives in order to reach eternal peace.
To evade detection or consequences, some killers resort to fires or explosions to carry out their attacks on the public.
In a twisted attempt to send a political message, some resort to mass murder.