The gunman dressed in black and wearing a helmet, body armour and a gas mask was surely a stunt for one of the summer's most anticipated films.
But as he emerged through a side door as the new Batman movie played on the screen, hurled gas canisters and sprayed the audience with bullets, screaming film-goers in Aurora, Colorado, realised the true horror taking place and dived for cover.
At least 12 people were killed and 58 wounded - 11 critically - at the midnight screening of The Dark Knight Rises on Friday - in one of the deadliest mass shootings in recent US history.
"He looked like an assassin ready to go to war," said Jordan Crofter, who was unhurt in the attack, which happened about 30 minutes after the film began.
The gunman, identified by police as 24-year-old James Holmes, used a military-style semi-automatic rifle, a shotgun and a pistol, stopping only to reload. The suspect marched up the aisle in the stadium-style multiplex cinema in the Denver suburb, picking off those who tried to flee, witnesses said. Authorities said he hit 71 people, at least one of them struck in an adjacent cinema by gunfire that went through the wall.
Within minutes, frantic emergency services calls brought 200 police officers, ambulances and emergency crews to the cinema. Holmes was captured in the car park and police said they later found that his flat nearby was booby-trapped.
Authorities gave no motive for the attack. The FBI said there was no indication of ties to any terrorist groups. In New York City, police commissioner Raymond Kelly said: "It clearly looks like a deranged individual. He has his hair painted red. He said he was the Joker, obviously the enemy of Batman."
Aurora police chief Dan Oates would not confirm that information, but did say he had spoken to Mr Kelly, with whom he used to work in New York. Asked whether Holmes had make-up to look like the Joker, Mr Oates said: "That to my knowledge is not true."
It was the worst mass shooting in the US since the November 5, 2009, attack at Fort Hood, Texas. A US Army psychiatrist is charged with killing 13 soldiers and civilians and wounding more than two dozen others. Friday's attack was the deadliest in Colorado since the Columbine High School massacre in suburban Denver in 1999, when two students killed 12 classmates and a teacher and wounded 26 others before killing themselves.
The new Batman movie, the last in the trilogy starring Christian Bale, opened worldwide yesterday with midnight showings in the US.