Police say Holmes, 24, dressed head-to-toe in protective tactical gear, set off two devices of some kind before spraying the Century 16 theater with bullets from an AR-15 rifle, a 12-gauge shotgun and at least one of two .40-caliber handguns police recovered at the scene.

Oates said the guns were purchased legally by Holmes at stores in the Denver area in the past two months. More than 6,000 rounds of ammunition were also purchased online, according to Oates.

A receipt obtained by CNN shows Holmes bought some of the tactical gear, including a vest and magazine pouch, online on July 2.

Oates said investigators are "confident" that Holmes acted alone.

The shooting unfolded inside a darkened theater packed with Batman fans, some in costume for the premiere of the movie.

Aurora police said how the suspect entered the theater is still under investigation.

A law enforcement source working the investigation told CNN that the gunman walked into the movie theater after purchasing a ticket.

After the movie was under way, he went out a rear exit door, propping it open, and gathered weapons before re-entering through the door, the source said.

As he re-entered, he tossed in a canister before starting to shoot, according to a second law enforcement source involved in the investigation and several witnesses.

Screaming, panicked moviegoers scrambled to escape from the black-clad gunman, who shot at random as he walked up the theater's steps, witnesses said.