The new Joker movie is now showing in theaters amid warnings from the FBI of potential violence.

These violence concerns trace back to 2012 when a gunman opened fire at a screening of The Dark Knight Rises in Aurora, Colorado. 12 people were killed and dozens more hurt.

Before this week's release of the Joker movie, an alert was sent out by the military, which contained intelligence from the FBI. It encouraged people to "identify two escape routes" in theaters and "run-- hide-- fight" if in an active shooting situation.

Thursday night, many theaters said they would not be accepting tickets after the film had started and law enforcement stepped up patrols.

Warner Brothers sent out a statement saying the movie is not "an endorsement of real-world violence of any kind."