Gunman complained about symptoms relating to post traumatic stress disorder, but experts say combat not always to blame.
A soldier with mental health issues killed three people and wounded 16 before turning the gun on himself.
A spate of violent incidents in Colorado and Maryland raise copycat theories as a possible explanation.
A shooting rampage in the heart of Washington claimed at least 12 lives, including the unidentified gunman.
A mother believes her son's psychic powers saved his life at the Sandy Hook school shooting, but psychology may explain it.