Many victims being treated in at least six hospitals were under 40, including a 6-year-old taken to Children's Hospital Colorado. The youngest person treated was a 4-month-old baby, who has been released. The oldest reported patient is 45.
Authorities said the gunman had appeared at the front of the theater during the film and released a canister of tear gas. Witnesses told reporters that the gunfire erupted during a shootout scene in "The Dark Knight Rises".
A suspect was apprehended in the shopping center's parking lot. He was named as 24-year-old James Holmes, two federal officials from different agencies told NBC News.
The incident occurred in the Century 16 Movie Theaters at the Aurora Town Center, police told NBC News. Aurora is a suburb less than 10 miles east of downtown Denver.
NBC station KUSA-Denver cited a witness as seeing a black-clad 6-foot-tall man wearing a riot helmet, goggles and bullet-proof vest.
However, many people attended the film dressed in Batman-related costumes.
Jennifer Seeger, who was inside the Aurora, Colo., movie theater where a gunman opened fire early Friday, describes the scene of the shooting as "mass chaos."
Witnesses said the gunman entered the theater through an emergency exit door.
The suspect was found in possession of a gas mask and ammunition was found in the suspect's car, police said.
The shooter had three weapons -- an assault-type rifle and two handguns, officials told NBC News. Holmes' car has Tennessee plates but authorities said he lived locally.