About 30 minutes into a midnight screening of the new Batman movie in Colorado, a gunman dressed in black wearing a helmet, body armour and a gas mask stepped through a side door.

For a moment, some audience members thought he was part of a stunt for one of the summer's most highly anticipated films.

But then he threw gas canisters that filled the packed suburban Denver theater with smoke, and opened fire as people screamed and dove for cover.

Twelve people were killed and 70 were injured in one of the deadliest mass shootings in recent U.S. history.

"He looked like an assassin ready to go to war," said Jordan Crofter, a moviegoer who escaped unhurt from the attack.

The shooting happened at around 12:30 a.m. MT at a multiplex theatre in a mall in Aurora, a community of about 327,000 people about 16 kilometres east of downtown Denver.

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James Holmes, 24, is the suspect in a shooting attack that killed 12 people in a suburb of Denver. (The University of Colorado/Reuters)

Aurora police Chief Dan Oates said police haven't been able to pinpoint how many shots were fired in the attack.

"There were many, many rounds fired," he said during a news conference on Friday afternoon.

Police, ambulances and emergency crews swarmed on the scene after frantic calls started flooding the 911 switchboard.

Witnesses shared dramatic accounts of the ordeal, from ducking the gunman's bullets to seeing lifeless bodies lying on the floor.

"There were bullet [casings] just falling on my head. They were burning my forehead," Jennifer Seeger said, adding that the gunman, dressed like a SWAT team member, fired steadily, stopping only to reload.  

"Every few seconds it was just: Boom, boom, boom," she said. "He would reload and shoot and anyone who would try to leave would just get killed."

Number of injured rises

Authorities had pegged the number of injured at 38 earlier in the day, but Oates revised the total upward on Friday evening.

'He said he was the Joker, obviously the enemy of Batman.' —New York City police Commissioner Raymond Kelly

Thirty patients remained in hospital, with 11 of them in critical condition, as of 3:30 p.m. MT, according to Colorado Gov. John Hickenlooper.

The youngest victim was a three-month-old baby, who was treated in hospital and later released.

Officers found the suspected gunman, who has been identified as 24-year-old James Holmes, next to his car behind the theatre. He is now in police custody.

The suspect wore a gas mask, a ballistic helmet and vest as well as leg, groin and throat protectors, Oates said, and had an AR-15 military-style semi-automatic rifle, a shotgun and a Glock pistol. A second Glock pistol was found in his car.

Holmes had legally purchased the firearms at local gun shops over the last 60 days, Oates told reporters at a Friday evening press conference. He had also ordered over 6,000 rounds of ammunition and multiple magazines over the internet, including a 100-round drum magazine for the rifle.

Firearms laws in Colorado

  • Permit to purchase rifles and shotguns? No
  • Registration of rifles and shotguns? No
  • Licensing of owners of rifles and shotguns? No
  • Permit to carry rifles and shotguns? No
  • Permit to purchase handgun? No
  • Registration of handguns? No
  • Licensing of owners of handguns? No
  • Permit to carry handguns? Yes for concealed.

Dealers are required to keep a record on the retail sale, rental, or exchange of handguns. The record should be open at all times to the inspection of an authorized police officer.

It is unlawful for any person convicted of a felony or conspiracy or attempt to commit a felony, or misdemeanour domestic violence or adjudicated delinquent for a felony to possess a firearm.

Source: National Rifle Association

Holmes is scheduled to make his first court appearance on Monday morning.

The shooting was the deadliest in the U.S. since an army psychiatrist killed 13 soldiers and civilians at Fort Hood, Texas, in 2009. In Colorado, it brought back memories of the shooting spree at nearby Columbine High School in 1999, in which two students opened fire and killed 12 classmates and a teacher.

New York City police Commissioner Raymond Kelly described the shooter as "a deranged individual" and said his hair had been painted red.

"He said he was the Joker, obviously the enemy of Batman," Kelly said.

Oates wouldn't confirm those details but said he had spoken to Kelly, with whom he used to work in New York.

Holmes graduated from high school in the San Diego area and had enrolled at the University of Colorado School of Medicine in 2011. He was "in the process of withdrawing" from the school's graduate neurosciences program, the university said in a statement.

There was no immediate word of any motive behind Friday's shooting, and Holmes' family issued a statement on Friday expressing their condolences to the victims and saying that they were co-operating with authorities.

"Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved," the statement said. "We are still trying to process this information and we appreciate that people will respect our privacy."

Aurora mental health services and the American Red Cross will be providing professional grief counselling to the local community, and a vigil is scheduled for Sunday.

Apartment booby-trapped

The suspect spoke of "possible explosives in his residence. We are dealing with that potential threat," Oates said.

Police used a hook-and-ladder fire truck to put a camera on a pole through a window in the suspect's third-floor apartment, which is located about six kilometres from the theatre. They found the apartment to be booby-trapped, prompting five buildings in the area to be evacuated as authorities attempted to disarm flammable and explosive material.

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Judy Goos hugs her daughter's friend, Isaiah Bow, 20, while eyewitnesses Emma Goos, 19, left, and Terrell Wallin, 20, right, gather outside a high school where people were brought for questioning Friday after the shooting. (Barry Gutierrez/Associated Press)

"It is a very vexing problem how to enter that apartment safely. I personally have never seen anything like what the pictures show us is in there," Oates said, adding that the apartment appears to contain trip wires and jars full of ammunition and liquids.  

Authorities have postponed further work at the residence until Saturday, when government resources will be dispatched to help figure out how to address the problem, Oates said.

'We heard people screaming'

Witness Hayden Miller told KUSA-TV that he was inside theatre 16 and heard several shots.

"Like little explosions going on and shortly after that we heard people screaming," he told the station.

Hayden said at first he thought it was part of a louder movie next door. But then he saw "people hunched over leaving [the] theatre."

Witnesses in other theatres reported seeing bullets come through the walls and hearing moans.

Speaking in Plano, Texas, Timothy Warner, the CEO of Cinemark, which owns the theatre where the shooting took place, called it "totally shocking."

'This is just an isolated.. deranged gunman that... had access to really high-power weapons.' —Timothy Warner, CEO of Cinemark

"Obviously, my immediate reaction was sadness and despair …" for the people who just went to the movie for an evening out, Warner said.

"Thousands of people went to the movies last night and had a great experience, " he said. "This is just an isolated.. deranged gunman that... had access to really high-power weapons."

Warner Bros., the Hollywood studio behind the highly anticipated film, said it is "deeply saddened to learn about this shocking incident."

"We extend our sincere sympathies to the families and loved ones of the victims at this tragic time," the company said.

The film's director, Christopher Nolan, issued a statement on behalf of the cast and crew, expressing their "profound sorrow at the senseless tragedy."

"Nothing any of us can say could ever adequately express our feelings for the innocent victims of this appalling crime, but our thoughts are with them and their families," Nolan said.

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U.S. President Barack Obama leads a moment of silence for victims of the Aurora, Colo., shooting on Friday. (Susan Walsh/Associated Press)

U.S. President Barack Obama called the shooting "horrific and tragic."

"As we do when confronted by moments of darkness and challenge, we must now come together as one American family," Obama said. "All of us must have the people of Aurora in our thoughts and prayers as they confront the loss of family, friends, and neighbours."

There was at least one cancellation of a scheduled screening of the movie in the wake of the shooting. The Friday premiere in Paris was scrapped.

The mass shooting also prompted some theatres and police around the U.S. to step up security at daytime showings of the film on Friday.

In Toronto, police said they had no plans to boost their staffing beyond usual levels at theatres showing The Dark Knight Rises.

With files from The Associated Press