"Everyone" knows video games cause school shootings...
The link between school shootings and violent video games played by minors has been firmly proven by the FBI, the Secret Service, the heads of six major health care organizations, the US Surgeon General, the American Psychological Association, Harvard, Indiana University, Michigan State, and even Vince Desi Desi (all caps removed because it's freaking annoying) JT

FBI & Secret Service: See Lies, damn lies, and statistics Basically they found no evidence of any such link, and only list video games amongst all violent media, as one of many indicators of possible aggressive tendencies (the USSS found only 12% of young school shooters had links to video games at all, and violent media was not found to be responsible for their violent attitudes)

6 health care orgs: The American Association of Pediatrics, the American Academy of Child & Adolescent Psychiatry, the American Psychological Association, the American Medical Association, the American Academy of Family Physicians, and the American Psychiatric Association released a joint resolution, stating that they are concerned about the amount of violent media minors are exposed to, since in theory it could cause changes in attitudes and possibly behavior, and called for more research. No links to school shootings are mentioned.

US Surgeon General: Youth Violence: A Report of the Surgeon General (2001) found that not enough studies have been done to conclude anything, and in particular, that sample sizes were too small. "The impact of video games on violent behavior remains to be determined."

American Psychological Association: Published two studies that indicate that playing violent video games make your grades suck, and cause you to honk horns more. From these conclusions they published a resolution with 5 other health care groups (see above)

Harvard: I suspect JT is confusing this with another university, because the Harvard study only talked about the reliability of ESRB content descriptors... Or maybe the study that found that watching violent TV correlates with less social activities in minors. Again, no "link to school shootings" (or video games, for that matter...)

Indiana University: Ah, the famous "playing games damages the brain" study. Except the study doesn't conclude this at all. It found that the amount of brain activity in the decision-making and behavioral control centres decreases over a long period of time as you watch more violent images. The study concludes that more research is necessary to find out what this means...

Michigan State: Researchers found that playing violent games results in increases in brain activity in the areas associated with aggressive cognitions, aggressive affects or aggressive behavior. No information is given as to what happens after the game is turned off.

Vince Desi Desi: Vince Desi, CEO of Running With Scissors, makers of the Postal series, has publicly stated that violence belongs on the screen, not the real world, and denounces any links between his games and real world events.