The city of New York absolutely prohibited the recreation of 9/11 destruction or chaos on location. The filmmakers were not even allowed to film actors looking upward toward where the towers would be. The drive of the officers up to the site was permitted to be filmed, but all scenes depicting events at or near the WTC were filmed in Los Angeles.
Fifty real-life members of PAPD, NYPD and FDNY (some of whom participated in the actual rescue) were flown to Los Angeles to film the chain of men moving stretchers out of the rubble.
Made with the full cooperation of the New York Port Authority, which allowed filming on location in their headquarters, and provided real uniforms and equipment for the production.
Test audiences believed that Dave Karnes (Michael Shannon), the former Marine, was a Hollywood invention. This is, in fact, a true story that occurred. Karnes later re-enlisted in the Marine Corps and served two tours of duty in Iraq. Also, as reported by the Associated Press on August 14, 2006, the Marine identified only as Sergeant Thomas (who disappeared shortly after 9/11) has been identified as Jason Thomas, a former Marine who grabbed his uniform and immediately drove to Ground Zero to help. Because his identity was unknown at the time of filming, Sgt. Thomas - who is black - was portrayed by William Mapother, a white man. The producers have since apologized to Sgt. Thomas for the miscasting. Thomas has laughed it off and told them not to be concerned, stating, "I don't want to shed any negativity on what they were trying to show."
In preparing for his portrayal of Sgt. John McLoughlin, Nicolas Cage focused on accurately speaking with a New York accent. And to also capture the fear and claustrophobia of McLoughlin's ordeal, Cage spent hours in a sensory-deprivation tank in Venice, California.