There were few answers Saturday to the motive of a suspect, who allegedly identified himself as "the Joker", and had opened fire during a screening of the new Batman movie at a Colorado movie theater.

Suspect, James E. Holmes allegedly killed 12 people and wounded 58 others, during a shooting rampage early Friday morning, at a showing of "The Dark Knight Rises".

24 year old Holmes, wearing a gas mask and body armour, threw tear gas canisters at movie-goers inside the Century Aurora 16 before randomly opening fire on the crowd.

It was reported that he apparently entered the cinema through an emergency exit door near the front of the auditorium.

Many people thought the gunshot sounds were coming from the film.

"After what seemed about two, three minutes of just constant shooting, we got up, I looked up and I saw he was either trying to reload a weapon, he started moving, so I realized that was a good time to get up. So I grabbed my girlfriend and my friend and we just started running down the stairs and then down the ramp. And while we were running down the stairs, there was just bodies on the floor, one guy was completely covered in blood, said one shooting witness.

"My buddy Chris he was up about 5 or 6 rows up from me, the dude right next to him, just, bullet right to the chest. He heard it hit and thunk, and blood all over his face, splattered," said an eye witness.

Holmes was arrested outside the cinema, armed with a rifle, a shotgun and two pistols.

With dyed red hair, he told police he was the Joker.

Meanwhile, investigations revealed that Holmes had extensively booby-trapped his apartment with explosives before the shooting spree.

According to police, Holmes had been extensively purchasing firearms for the past 2 months.

"In the last 60 days he (suspect) purchased four guns at local Metro Gun Shops and through the internet he purchased over six-thousand rounds of ammunition," said Dan Oates, Aurora Police Chief.

Few details have emerged about Holmes, apart from having dropped out of a neuroscience program at the local medical school.

"He was an honours student. So academically, he was at the top of the top. He really distinguished himself from an academic point of view during our four years with us, graduating with highest honours," said Timothy White, University of California Riverside Chancellor.

Prior to the shooting, Holmes's only brush with the law had been a summons for speeding.

Police have ruled out any other suspects.

A candlelight vigil has been held in honour of the victims.