Police in the U.S. state of Colorado say 12 people were shot to death and 59 wounded in a shooting at a movie theater in a suburb of Denver.

Authorities say the midnight premier of “The Dark Knight Rises,” the latest Batman movie, had just begun at the Century 16 theater in the city of Aurora, when a masked man dressed in black and wearing a bulletproof vest faced the packed crowd from the front of the theater. They say he set off canisters spewing an irritant and smoke and then started randomly shooting. Witnesses say he fatally shot an infant at point-blank range.

Police say they arrested the shooter a short time later in the parking lot outside the theater, and recovered an assault rifle, a shotgun, a handgun and a gas mask.

Hours after the incident unfolded Friday in the darkened movie theater, police said they found the gunman's apartment residence “booby-trapped” with explosives and chemical devices. They evacuated nearby residents.

Law enforcement officials identified the suspect as James Holmes, a 24-year-old American who was in the process of dropping out as a neuroscience graduate student at a nearby university. They said there was no indication the shooting was tied to any terrorist group.

U.S. President Barack Obama, abandoning a planned re-election campaign rally in Florida, called on Americans to support the victims of the carnage. He said “such violence, such evil is senseless” and that the nation is “heartbroken.”

Mr. Obama said the mass shooting was inexplicable.

“We may never understand what leads anybody to terrorize their fellow human beings like this.”

Mr. Obama's Republican presidential challenger, Mitt Romney, also stopped campaigning. He called the incident “a hateful act” and said the hearts of everyone “break with the sadness of this unspeakable tragedy.”

The movie theater attack was the worst mass shooting in the United States since 2009 when an Army psychiatrist killed 13 soldiers and civilians at a military base in Texas.

Aurora Chief of Police Daniel Oates told a news conference police are “confident” that only one man was involved, contrary to initial reports that there may have been an accomplice.

The police chief said the suspect told investigators he had multiple explosives at his residence in a North Aurora apartment building, and investigators later found the apartment booby-trapped with flammable and explosive materials. Residents at five nearby buildings were ordered out of their residences as authorities sought to disarm the dangerous site.

“The suspect who's in custody, made statements about possible explosives in possession of his residence. At this time, there is a residence in North Aurora — its an apartment apartment building — that we have evacuated and we are dealing with as potentially there are explosives inside there. We're also concerned about explosives in the parking lot, hence the parking lot is completely sealed off. We have all the area bomb squad teams.”

One patron at the theater, Jaime Marshall, described a chaotic scene in the moments after the shooting, and said she saw a girl with a bullet wound in her leg.

“So we went up those stairs and we looked to the wall, to the side of the theater where we heard the sounds from and there were bullet holes and so that's when we started screaming for all our friends just to get out. We went up the stairs and we were looking over the lobby and police were out there. There was one officer in a gas mask holding a huge rifle and they were just waving people, trying to get people through the lobby as fast as possible so we ran down the stairs, we ran out of the theatre. We walked out, I saw a girl with a bullet in her leg.”