At least a dozen people were killed and 50 injured during a shooting spree at a mall movie theater in Aurora, Colo., police authorities said.
The massacre took place during a midnight showing of the latest Batman movie, The Dark Knight Rises.
The gunman, who is now in police custody, reportedly released a gas canister before opening fire into the multiplex theater at about 12:30 a.m. MDT.
"Witnesses tell us he released some sort of canister. They heard a hissing sound and some gas emerged and the gunman opened fire," Aurora Police Chief Dan Oates said at a news conference.
"I told my friend, 'We've got to get out of here,' but then he shot people trying to go out the exits," Jennifer Seeger told NBC's Today show.
Police later found the suspect near a car behind the movie theater. His name has not yet been released but he's said to be in his 20s.
"A gas mask, rifle, handgun, at least one additional weapon (were) found inside," Aurora police spokesman Frank Fania said.
Jacqui Montgomery, a doctor at the University of Colorado Hospital. She told the BBC she's seen a wide range of injuries.
"Right now we have seen a variety of patients coming into the hospital," she said. "We've been seeing them for well over an hour. We've had casualties coming in with a variety of different injuries, less critical to severe."
"One of our highest priorities will be dealing with the families of the victims," Oates said. "Ten people died in the theater and four died in hospital as a result of their injuries."
Meanwhile, President Barack Obama said he was "shocked and saddened" by Friday's tragedy and is pledging his support to the victims' families.
"My administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time," the president said in a statement. "We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded."
GOP presumptive nominee Mitt Romney also offered his condolences, calling the massacre an act of "senseless violence."
"We are praying for the families and loved ones of the victims during this time of deep shock and immense grief," he said.
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