AURORA, Colo. (AP) -- 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.