Oregon Health & Science University psychiatrist Dr. Jerald Block has researched the 1999 Columbine High School shootings in Colorado, which resulted in the deaths of 15 people, including the two students who initiated the attack, Eric Harris and Dylan Klebold.

Prior to the shootings, both teens spent a significant amount of time playing first-person-shooter computer games. Block suggests that these virtual worlds became essential for the teens and that Harris and Klebold may have been unable to distinguish the boundaries between their virtual lives and their real lives -- in effect mixing the two.

"On one hand, virtual worlds allow people to feel connected and empowered ... to escape stress and have an outlet for aggression," Block said in a statement. "On the other hand, when a heavy user must eliminate or cut back on the virtual, as was the case with these two killers at times, the user can feel lonely, anxious, or angry. When a person goes 'dry,' the situation can turn dangerous."

Block presented his findings at the annual meeting of the American Psychiatric Association in Washington.