This optional level titled "No Russian" has the player assume control of a deep-cover CIA agent, joining a group of Russian Ultra Nationalist terrorists enacting an airport massacre. The name comes from a briefing from the terrorist leader, Vladimir Makarov, who tells the player's character and the other gunmen not to speak any Russian throughout the scene. Civilians scream and run in terror, the injured crawl away leaving a trail of blood, and some try to drag others to safety.
While not an ideal simulation, as civilians do not play dead and aren't overly creative at hiding, it does provide some degree of mental preparation for a spree killing terrorist attack. Judge for yourself.
There's no mention of this Call of Duty mission during the trial, and Breivik probably didn't want to mention it himself as psychologists would be quick to fantasize about the possibility that Breivik wanted to play out his favorite modern warfare mission in real life.