James Holmes, 24, has been held in solitary confinement awaiting his hearing, where the charges of suspicion of first-degree murder will be read against him. Friday's shootings left 12 dead and 58 wounded, some critically.
"He's not talking to us," Aurora Police Chief Dan Oates said.
Police have said Holmes began buying guns at Denver-area stores nearly two months before the shooting and that he received at least 50 packages in four months at his home and at school. He recently purchased 6,000 rounds of ammunition over the Internet, the chief said.
During the attack, Holmes allegedly set off gas canisters and used a semiautomatic rifle, a shotgun and a pistol to open fire, Oates said.
The semiautomatic assault rifle jammed during the attack, forcing the gunman to switch to another gun with less firepower, a federal law enforcement official told The Associated Press. That malfunction and weapons switch might have saved some lives.
The owner of a gun range told The Associated Press that Holmes applied to join the club last month but never became a member because of his behavior and a "bizarre" message on his voice mail.
When Lead Valley Range owner Glenn Rotkovich called to invite Holmes to a mandatory orientation the following week, he said he heard a message on Holmes' voice mail that was " guttural, freakish at best."
He eventually told his staff to watch out for Holmes at the July 1 orientation and not to accept him into the club, Rotkovich said.
Officials at the University of Colorado Anschutz Medical Campus were looking into whether Holmes, a former doctoral student in neuroscience, used his position in a graduate program to collect hazardous materials.
Holmes' reasons for quitting the doctoral program in June, just a year into the five- to seven-year program, remained a mystery. He recently took an intense, three-part oral exam that marks the end of the first year. Those who do well continue with their studies and shift to full-time research, while those who don't do well meet with advisers and discuss their options. University officials would not say if he passed, citing privacy concerns.
Ritchie Duong, a friend who has known Holmes for more than a decade, told the Los Angeles Times that he last saw Holmes in December and his friend seemed fine.
Academics came easily to Holmes, Duong said. "I had one college class with him, and he didn't even have to take notes or anything."
The family's pastor recalled a shy boy who was driven to succeed academically.
"He wasn't an extrovert at all. If there was any conversation, it would be because I initiated it, not because he did," said Jerald Borgie, who last spoke with Holmes about six years ago.
Sunday was a day for healing and remembrance in Aurora. Several thousand people attended a prayer vigil, and President Barack Obama visited with families of the victims.
Obama said he told the families that "all of America and much of the world is thinking about them."