(CNN) -- A heavily armed gunman tossed tear gas into the darkness of an Aurora, Colorado movie theater Friday and then sprayed the audience with gunfire during an early morning screening of the new Batman movie, killing 12 and wounding 38 others, authorities said.
One of the injured was just 3 months old, according to hospital workers.
Police arrested a man believed to be the shooter in a rear parking lot of the theater, Frank Fania, a police spokesman, told CNN. The suspect was not immediately identified. Police said he was a man in his 20s from Aurora.
"He did not resist. He did not put up a fight," Fania said. Police seized a rifle and a handgun from the suspect, and another gun was found in the theater, he said.
KMGH: Mass shooting inside Aurora movie theater
Oates said there was no evidence of a second gunman, and FBI spokesman Jason Pack said it did not appear the incident was related to terrorism.
President Barack Obama said he and first lady Michelle Obama were "shocked and saddened" by the shooting and pledged the administration's support for victims of the shooting.
"As we do when confronted by moments of darkness and challenge, we must now come together as one American family," it said.
Chaos broke out during the showing of "The Dark Knight Rises" at the Century Aurora 16 theater when the shooting began, police and witnesses said.
Police said the gunman "appeared" in the front of the theater and threw a smoke bomb before opening fire. A federal law enforcement official told CNN the smoke bomb described by witnesses was tear gas. It was unclear how the man got into the theater, but witnesses told CNN affiliate KUSA that he came in through an emergency door.
One movie-goer, who was not identified, told KUSA the gunman was wearing a gas mask.
KUSA: One person in custody
Some people in the audience thought the thick smoke and gunfire was a special effect accompanying the movie, police and witnesses said.
"We just heard a pop, pop, pop, pop," said Quentin Caldwell, who was attending the Batman showing in an adjacent theater.
Cell phone video taken by someone at the theater showed scores of people screaming and fleeing the building. Some had blood on their clothes.
One police officer carried a girl believed to be about 9 with gunshot wounds to her back out of the theater, a witness said. "She wasn't moving."
Officers rushed many of the wounded to hospitals in their patrol cars.
Authorities also have evacuated the suspect's Aurora apartment building after "he made a statement about explosives" in his unit, Oates said.
The shooter used at least four guns -- an "AK type" rifle, a shotgun and two handguns, the federal law enforcement official told CNN.
Bureau of Alcohol, Tobacco and Firearms agents have searched the suspect's car and went to his home to search for explosives, agency spokesman Tom Mangan said.
Law-enforcement officers who searched the the suspect's apartment found "items of interest," a federal law enforcement source involved in the investigation told HLN. He did not elaborate.
A woman who lives across the street from where the suspect is believed to live said her building had been evacuated by police around 4 a.m.
"They told us there was a bomb or bomb material located in the house across the street from us." Rebecca Bradshaw said.
In addition to looking into the possibilty of explosives, ATF agents also are conducting emergency traces on the weapons used to see how they were obtained, Mangan said.
KWGN: Witnesses first thought gunfire was part of movie
Police initially said 14 people had died -- 10 in the theater and four at area hospitals, but revised the death toll to 12 later Friday morning, according to Lanigan.
Several people remained in critical condition at area hospitals, however.
Of the wounded, at least 20 were being treated at the University of Colorado Hospital, said spokeswoman Jacque Montgomery. All of the wounded suffered from gunshot wounds, which ranged from minor to critical, she said.
"They're arriving by police, by ambulance. Some are walking in," she said.
The victims being treated there ranged in age from 3 months to 45, the hospital said.
Denver Health Center had six patients from the shooting, one of them in critical condition, five in fair, said Shelly Davis, house supervisor.
Swedish Health Center spokeswoman Nicole Williams said her hospital was treating three people, two of them in critical and one in fair condition. A fourth patient was on the way, she said.
Parker Adventist Hospital was treating two people for minor injuries, according to a spokeswoman.
Hundreds of police officers descended on the theater, and the FBI has joined the investigation.
"We were calling for help from every police and fire agency," Fania said.
Aurora, a Denver suburb, is about 13 miles from Littleton, Colorado -- site of the April 1999 Columbine High School massacre.
In that incident, two teenage students, Dylan Klebold and Eric Harris, armed themselves with guns and bombs and opened fire inside the high school. They killed 13 people and wounding 23 others before killing themselves.
Are you there? Share your story with us on CNN iReport.
CNN's Ed Payne, Joe Sterling, Tina Burnside, Mike Brooks and Jim Spellman contributed to this report.
Bei einer Schießerei in einem Kino in Aurora bei Denver, der Hauptstadt Colorados in den Vereinigten Staaten, wurden am Freitag gegen 0:30 Uhr Ortszeit mindestens zwölf Personen getötet, 38 weitere wurden verletzt. Die meisten der Opfer starben an Ort und Stelle, vier von ihnen erlagen ihren Schussverletzungen im Krankenhaus. Zunächst hieß es, der Schütze habe 14 Personen tödlich getroffen, doch am frühen Morgen in Denver korrigierte die Polizei die Zahl der Opfer um zwei nach unten.

Die Polizei nahm den Tatverdächtigten fest, der keine Gegenwehr leistete. Nach den Angaben von Frank Fania von der Polizei in Aurora sei das Motiv des Schützen noch unklar. Der Mann habe angegeben, in seiner Wohnung Sprengsätze deponiert zu haben. Das Gebäude und seine Umgebung wurden evakuiert. Der Verdächtige habe eine schusssichere Weste getragen und sei mit einem Gewehr sowie zwei Pistolen bewaffnet gewesen. Die Schüsse fielen während der lokalen Premiere des neuesten Batman-Filmes „The Dark Knight Rises“. Zahlreiche Kinobesucher haben die Schüsse zunächst für einen Bestandteil des Films gehalten. Der mutmaßliche Täter, so sagte Fania dem Nachrichtensender CNN, habe zunächst Tränengas in den Kinosaal geworfen und dann das Feuer eröffnet. Die Festnahme des mutmaßlichen Täters erfolgte auf einem Parkplatz hinter dem Kino. Nach Angaben von CNN gibt es keinen terroristischen Hintergrund für die Tat. 

„Michelle und ich sind schockiert und betroffen von dem schrecklichen Angriff in Colorado“, reagierte US-Präsident Barack Obama in einer Erklärung und forderte dazu auf, für die Betroffenen zu beten und ihnen beizustehen. Der Ort des Geschehen liegt etwa 20 Kilometer entfernt von Littleton, wo im April 1999 bei einem Massaker an der Columbine High School zwei Schüler 13 Menschen töteten und 23 weitere verletzten, bevor sie sich selbst erschossen.