Mayor Michael R. Bloomberg of New York, who has waged a national campaign for stricter gun laws, offered a political challenge. “Maybe it’s time that the two people who want to be president of the United States stand up and tell us what they are going to do about it,” Mr. Bloomberg said during his weekly radio program, “because this is obviously a problem across the country.”

Luke O’Dell of the Rocky Mountain Gun Owners, a Colorado group on the other side of the debate over gun control, took a nearly opposite view. “Potentially, if there had been a law-abiding citizen who had been able to carry in the theater, it’s possible the death toll would have been less.”

Some survivors thought at first they were witnessing a promotional stunt. The gunman, wearing what Aurora Police Department officials described as nearly head-to-toe “ballistic gear,” including a throat protector and leggings, plus a gas mask and a long black coat, came in through a parking lot exit door near the screen of Theater 9.

“He walked in so casually,” said a witness, Jordan Crofter, 19, a Batman fan who had gone with a group of friends and had a seat in the front row. The gunman, still perhaps regarded by some as a performer, then released two devices down the theater aisles emitting what the police said was smoke or some sort of irritant.

Witnesses told the police that Mr. Holmes said something to the effect of “I am the Joker,” according to a federal law enforcement official, and that his hair had been dyed or he was wearing a wig. Then, as people began to rise from their seats in confusion or anxiety, he began to shoot. The gunman paused at least once, several witnesses said, perhaps to reload, and continued firing.

Mr. Holmes was detained by the police soon afterward, standing by his white Hyundai. He was identified by the authorities as a former Ph.D. student at the University of Colorado in Denver, and an honors graduate in neuroscience from the University of California, Riverside. He had in the car an AR-15 assault rifle, a Remington 12-gauge shotgun, and a .40 caliber Glock handgun, said Chief Dan Oates of the Aurora police, and all three were believed to have been used inside the theater. Another Glock .40 caliber handgun was recovered inside the theater. Chief Oates said that “many, many” rounds were fired, but that there was no count so far.