Police said the complicated setup at the Aurora home of James E. Holmes was meant to harm or kill anyone who entered, CNN reported Tuesday.
"Imagine that fireball ... you would have an explosion that would knock down the wall of [nearby] apartments," the official said. "That flame would have consumed the entire third floor" of the apartment complex.
Firefighters would have arrived at a building "that would have been completely consumed in flames," the official said.
Holmes made his first court appearance Monday. He is accused in the deaths of 12 people and the wounding of 58 others Friday when he allegedly opened fire on theater-goers at a midnight showing of the latest Batman movie, "The Dark Knight Rises."
He will be formally charged next Monday.
The official, who was not identified, told CNN the grenades were wired to a control box in the kitchen that bomb technicians disabled aided by a remote-control robot that put water on it.
"It looked like spaghetti," said the official, adding that the control box was sent to the FBI laboratory in Quantico, Va., for forensic analysis.
The Century 16 Movie multiplex will be closed until at least Wednesday so police can complete their investigation and Holmes' defense team can access the scene Tuesday.
Four of those killed were members of the U.S. military. As of late Monday, at least 15 people remained hospitalized, five in critical condition.
Lisa Damiani, an attorney representing the Holmes family but not the suspect, said family members were not revealing their location, CNN reported.
"They're doing as well as they can, under the circumstances," Damiani told reporters at her office in San Diego, Calif., where the family lives. "I think everyone can imagine how they're feeling -- anyone who's ever been a parent."
However, she said, the family "has elected not to discuss James or their relationship with James at this time."
Meanwhile authorities elsewhere in the United States said they've had several incidents tied to screenings of "The Dark Knight Rises."
In Maine, police said a man stopped for speeding during the weekend with a number of guns, including an assault rifle, and boxes of ammunition in his vehicle, told authorities he took a loaded gun into a showing of the Batman movie.
State Police said Timothy Courtois, 49, of Biddeford also had news clippings of the recent Aurora, Colo., movie slayings in his car, The Boston Globe reported Monday. Courtois told troopers he was traveling to shoot his former employer.
A Department of Public Safety spokesman said Courtois didn't indicate he had a plan to use the weapon inside the theater.
The Los Angeles County Sheriff's Department said it arrested a man at a Norwalk, Calif., theater showing of "The Dark Knight Rises" after the man allegedly made threats about a gun, the Los Angeles Times reported Tuesday.
The suspect, Clark Tabor, was arrested on suspicion of making criminal threats.
Tabor allegedly grabbed his cellphone Sunday evening in the cineplex, shouting, "Does anyone have a gun?" and "I should go off like in Colorado," the Los Angeles County Sheriff's Department said in a statement.
A confrontation with an intoxicated man at a Sierra Vista, Ariz., theater prompted movie patrons to flee a showing of "The Dark Knight Rises" the same day a gunman killed 12 people in Aurora, the Tuscon Arizona Daily Star reported.
Police said witnesses reported Michael William Borboa, 23, appeared to be intoxicated and was acting strangely during the screening.
Some patrons confronted the man, causing "mass hysteria" and prompting about 50 people to flee, police said. Police said he brought to the theater a backpack that held an empty alcohol bottle and a half-filled bottle of alcohol.
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