According to the Denver Post, police were unable to enter the third-floor apartment yesterday because of fears that it was booby-trapped with trip wires and explosives.
"We have been successful in defeating our first threat," police spokeswoman Cassidee Carlson told reporters. "This is some serious stuff they're dealing with."
There are thought to be other devices inside the apartment, which technicians may have to detonate in a controlled explosion. That risks destroying valuable evidence, however, Carlson said.
Police were able to photograph the interior using a camera attached to a pole and inserted through a window. The pictures revealed "trip wires, jars full of liquid and things resembling mortar rounds," according to the Post.
Aurora Police Chief Dan Oates told reporters that he had never seen anything like it, according to CBS News.
Holmes, 24, was taken into custody in a parking lot behind the movie theater where he allegedly shot 70 people, at least 12 of them fatally, early Friday morning.
He warned police that his apartment contained explosives, but since then is said to have stopped cooperating with authorities. He also reportedly calls himself "the Joker."
Former classmates and neighbors told NBC News they remembered Holmes as a quiet, intelligent and unassuming person.