The shooting unfolded inside a darkened theater packed with Batman fans, some in costume for the premiere of the movie Batman "The Dark Knight Rises" at Aurora, Colorado, USA Last July 19, 2012.
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.
At least 12 people were killed in the rampage and 58 were injured, one fewer than earlier reported. Of those injured "nearly everyone was shot," Aurora Police Chief Dan Oates said, adding that a handful of people were hurt in the resulting chaos.
Ten of the victims were killed inside the theater while two others died at hospitals. As of Friday evening, the 10 bodies had been removed from the complex.
The gunman named James Holmes was wearing a ballistic helmet and protective gear for his legs, throat and groin, black gloves and a gas mask. He told police he was "the Joker," according to a federal law enforcement source with detailed knowledge of the investigation.
He was charged with 24 counts of first-degree murder -- two counts for each of the 12 people killed in the shooting.
Twelve of the murder counts cite "deliberation," and 12 cite "extreme indifference" to the value of human life.
The 24-year-old former doctoral student also was charged with 116 counts of attempted murder -- two for each of the 58 moviegoers wounded in the attack. Finally, he was charged with one count of felony possession of explosive devices and one count related to the use of an assault weapon, a shotgun and a handgun during the incident.
The 142 counts are all in connection with the July 20 massacre in the Century Aurora 16 multiplex minutes into a midnight screening of "The Dark Knight Rises."
Shackled around his wrists and ankles, Holmes was escorted into Arapahoe County Courthouse by two sheriffs deputies. Five other sheriffs deputies were standing in the courtroom.
It's results of cancellation of other screening of Batman "The Dark Knight Rises".
