Police were trying to enter the home of man suspected in the deaths of at least 12 people killed when a gunman in a gas mask opened fire about 12.30am in a cinema showing the new Batman movie in Aurora, Colorado.
The man was arrested in a car at the shopping mall that housed the theatre screening The Dark Knight Rises in the Denver suburb, Police Chief Dan Oates told reporters.
Authorities believe that the apartment of James Holmes (24), is "currently booby-trapped," Mr Oates said.
He said pictures taken by remote devices "look pretty disturbing." Authorities seized two .40 calibre handguns, one 12-gauge shotgun and one .223 calibre rifle, said a federal law- enforcement official who was not authorised to speak publicly.
The shooting, which wounded dozens, was the worst in the US since 13 soldiers and civilians were killed and 43 were wounded when a gunman opened fire at Fort Hood, Texas.
"We have no evidence of additional shooters," Mr Oates said at an earlier briefing. "The gunman was found in a car in the parking lot with a rifle, handgun, gas mask." Holmes was a graduate student in neuroscience at the University of Colorado-Denver and was in the process of withdrawing, the Associated Press reported.
He had no known ties to terrorists, federal sources said.
Holmes attended high school in San Diego County, California, where his parents and other relatives still live, according to the U-T San Diego newspaper.
"Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved," the family said in a statement.
The Dark Knight Rises is rated PG-13 and there were many children, including some in costumes, at the Century 16 Movie Theaters at the Aurora Town Center.
The maker of the Batman movie, Time Warner, cancelled the film's Paris premiere after the shooting and issued a statement of sadness and sympathy for the victims.
In New York City, Police Commissioner Raymond W. Kelly deployed officers to theatres showing the movie.