Friends of the gunman in the deadly shooting spree Monday at a Washington Navy Yard remember him as a nice guy with flashes of a temper and an obsession with violent video games.

Aaron Alexis, the gunman who killed 12 in the rampage, was liked by neighbors, but he was known to immerse himself in violent video games for hours on end, one of his neighbors told the Dallas Morning News.

Nutpisit Suthamtewakul, the owner of the Happy Bowl Thai restaurant in Fort Worth, Texas, recalled Alexis as skilled at these games. Alexis would play marathon sessions for hours, The Wall Street Journal reported. Another friend said that Alexis would play first-person shooting games online. These games would be so time consuming, that friends would bring Alexis food during these binges.

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While some neighbors and acquaintances described him as "nice," his father once told detectives in Seattle that his son had anger management problems related to post-traumatic stress brought on by the terrorist attacks of Sept. 11, 2001. He also complained about the Navy and being a victim of discrimination.

Several other mass killers, including Columbine shooters Eric Harris and Dylan Klebold and Norwegian mass murderer Anders Breivik, have been linked to violent video games. And some experts worry that as the games get more violent and more realistic, so does their power to blur the line between fantasy and reality in alienated gamers.

“More than any other media, these video games encourage active participation in violence," Bruce Bartholow, an associate professor of psychology at the University of Missouri, who has studied the issue, told Fox News earlier this month. “From a psychological perspective, video games are excellent teaching tools because they reward players for engaging in certain types of behavior. Unfortunately, in many popular video games, the behavior is violence.”

The Associated Press contributed to this report