NEW YORK, Jul 20 (Reuters) - A gunman in a gas mask killed 12 people and injured dozens more at a midnight showing of the new "Batman" movie in a Denver, Colorado, suburb early on Friday.

Here is a timeline of the events, based on the accounts of witnesses, police and a law enforcement source close to the investigation:

* Just before midnight, hundreds of people packed into four sold-out screenings of the Batman movie, "The Dark Knight Rises," at the Century 16 Movie Theaters in Aurora, Colorado. One, an aspiring sportscaster who went by the name Jessica Redfield, sent a tweet: "Of course we're seeing Dark Knight." She had missed by minutes being on the scene of another shooting rampage just a month before in Toronto. She would be one of 12 killed in Aurora.

* The suspect, James Holmes, a 24-year-old who had recently dropped out of a PhD program in neuroscience at the University of Colorado, purchased a ticket, and went into theater No. 9.

* The screening began at midnight. It is not clear how many people were seated in the theater, but police said later there were at least 200 witnesses.

* The suspect approached an emergency exit to the right of the screen and went out, leaving the door propped open, and geared up. Then, clad in black ballistics and tactical gear, he emerged with three weapons: an AR-15 assault rifle, a Remington 870 shotgun and a .40-caliber Glock handgun.

* As he stood in front of the screen, some in the audience said they believed he was part of the show. He then released a canister that let out a hissing sound, and began taking aim at members of the audience. One witness, 22-year-old Jennifer Seeger, was seated in the second row of the theater and said she watched as the shooter took aim at her. She dove under the seat in front of her and heard a woman scream behind her.

* Local police began receiving emergency calls at 12:39 a.m. Within a minute, hundreds of 911 calls had been placed.

* Police arrived at the scene at 12:40 a.m. (0640 GMT) The suspect, Holmes, was taken into custody in the parking lot near the back door of the theater. In his car, a white Hyundai, police found a fourth weapon, another .40-caliber Glock handgun.

* The attack killed 12 people and wounded 59 others, including children, hospital officials and police said.

(Reporting by Ellen Wulfhorst, Joseph O'Leary, Tim Gaynor, Sunaina Karkarey, Jim Wolf and Cynthia Johnston and Edith Honan; Editing by Jackie Frank)