AURORA, Colo. -- A law enforcement official says investigators have found a Batman mask inside the apartment of a man suspected of opening fire at a midnight premiere of "The Dark Knight Rises."

The official spoke to The Associated Press on Sunday on condition of anonymity because he was not authorized to speak to the news media.

Authorities say they cleared dangerous materials from the apartment of 24-year-old James Holmes over the weekend after he allegedly set booby traps inside.

Holmes was arrested Friday after authorities say he set off gas canisters and shot unsuspecting movie-goers in an attack that killed 12 people and injured dozens of others.

Aurora Police Chief Dan Oates says it could take months to determine a motive and police are working with FBI behavioral analysts.

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  • Colorado Community Mourns In Aftermath Of Deadly Movie Theater Shooting

    AURORA, CO - JULY 24: A Federal Bureau of Investigation agent takes photos as he investigates inside the apartment where suspect James Holmes lived July 24, 2012 in Aurora, Colorado. Holmes, 24, is accused of killing 12 people and injuring 58 in a shooting spree on July 20 during a screening of 'The Dark Knight Rises.' According to police Holmes' apartment had more than 30 homemade grenades and 10 gallons of gas inside that was booby-trapped to go off. (Photo by Joshua Lott/Getty Images)