Law enforcement officials also revealed that Holmes, 24, has not been cooperating with them and that it could take months to learn what prompted the attack early Friday on a packed theater of moviegoers watching the premiere of the latest Batman movie. The assault killed 12 and left 58 wounded.
Investigators found a Batman mask inside Holmes' apartment after they finished clearing the home of booby traps and ammunition, a law enforcement official close to the investigation said Sunday on condition of anonymity because he was not authorized to speak to the press.
Meanwhile, President Barack Obama flew to Colorado for a few hours to comfort residents in a state that's critical to the November election. He began his visit with the family members of the victims at the University of Colorado Hospital, which treated 23 of the people injured; 10 remain there, seven hurt critically. The hospital is a short drive from the site of the shooting.
After meeting with the families, he said that he was there "not as president but as a father and a husband."
He said that "we can all understand what it would be to have someone taken from us in this fashion."
Holmes was being held in solitary confinement at a Denver-area county detention facility, Aurora Police Chief Dan Oates said, and is "lawyered up."
"He's not talking to us," the chief said. He is scheduled for an initial hearing Monday at 9:30 a.m. MDT, and has been assigned a public defender.
Police have said that Holmes began buying guns at Denver-area stores nearly two months before Friday's shooting and that he received at least 50 packages in four months at his home and at school. Also on Sunday, a gun range owner east of Denver said he recently rejected a membership application from Holmes in part because of a bizarre voice mail greeting on Holmes' phone.
While the University of Colorado disclosed that it was cooperating with police in the case, that disclosure was one of the few the university has made three days after the massacre. It remained unclear whether Holmes' professors and other students at his 35-student Ph.D. program noticed anything unusual about his behavior.
His reasons for quitting the program in June, just a year into the five- to seven-year program, also remained a mystery.
Holmes 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, including retaking the exam. University officials would not say if he passed, citing privacy concerns.
The university said Holmes gave no reason for his withdrawal, a decision he made in June.
Holmes was not allowed access from the institution after his withdrawal, which was "standard operating procedure" because he was no longer affiliated with the school, Montgomery said. Holmes had no contact with university police, she said.
The university declined to release any details of his academic record, citing privacy concerns, and at least two dozen professors and other staff declined to speak with The Associated Press. Some said they were instructed not to talk publicly about Holmes in a blanket email sent to university employees.
Jacque Montgomery, a spokeswoman for the University of Colorado medical school, said that police have told the school to not talk about Holmes. The university also took down the website for its graduate neuroscience program on Saturday.
Dan Keeney, president of DPK Public Relations in Dallas, said asking for silence from university employees because of a police investigation was appropriate, but taking down the website was "indefensible" for a publicly funded university unless the school believed it contained inaccurate information relating to the program.
"It's an indefensible action," he said. "It's disappointing to hear that they would take that action because it suggests that it's not in the public's interest to have access to that information and I think it is in the public's interest."
Amid the continuing investigation of Holmes and his background, Sunday was a day for healing and remembrance in Aurora, with President Barack Obama arriving to visit with families of the victims and a vigil that began in the early evening.
Obama said he told the families of the victims of Friday's massacre that "all of America and much of the world is thinking about them." He met with them at the University of Colorado Hospital in Aurora, which treated 23 of the people injured in the mass shooting; 10 remain there, seven hurt critically.
Congregations across Colorado prayed for the shooting victims and their relatives. Churches sent out social-media appeals for neighbors who wanted to join in remembrance. Elderly churchgoers at an aging Presbyterian church within walking distance near Holmes' apartment joined in prayer, though none had ever met him.
Hundreds gathered for prayers and healing at the vigil Sunday night, where a banner said, "Angels Walk With Those Who grive.
"You're not alone, and you will get through it," said the Rev. Kenneth Berve, pastor at Grant Avenue United Methodist Church and a witness to Friday's horrors. "We can't let fear and anger take control of us."
Aurora resident Heather Lebedoff, 24, placed a rose on each cross that had been erected in memory of those killed. She said she didn't know anyone in the theater, but she felt connected to her neighbors and all the pain they have gone through.
"This is the city I live in, and I know there are a lot of people affected by this. Stuff like this really shows what love and community is all about."
Meanwhile, the owner of a gun range told the AP 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.
He emailed an application to join the Lead Valley Range in Byers on June 25 in which he said he was not a user of illegal drugs or a convicted felon, said owner Glenn Rotkovich. When Rotkovich called to invite him to a mandatory orientation the following week, he said he heard a message on Holmes' voice mail that was "bizarre - guttural, freakish at best."
He left two other messages but eventually told his staff to watch out for Holmes at the July 1 orientation and not to accept him into the club, Rotkovich said.
Ritchie Duong, a friend who has known Holmes for more than a decade, told the Los Angeles Times that in high school he liked to play cards and video games. They both attended undergraduate school at the University of California, Riverside, where they saw each other once a week to watch the TV show "Lost."
Duong last saw Holmes in December when they met for dinner in Los Angeles and saw a movie together. His friend seemed fine, he told the newspaper. Academics came easily to Holmes both at high school and at the UC Riverside, Duong said.
"I had one college class with him, and he didn't even have to take notes or anything. He would just show up to class, sit there, and around test time he would always get an 'A,'" said Duong, 24.
The pastor for the family of the suspect also 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, senior pastor of Penasquitos Lutheran Church. Borgie said he never saw the suspect mingle with others his age at church.
Holmes told the pastor he wanted to attend a University of California school and pursue graduate studies. Borgie, who last spoke with Holmes about six years ago, doesn't remember the suspect being more specific about his goals.
"He had some goals. He wanted to succeed, he wanted to go out, and he wanted to be the best," Borgie said. "He took pride in his academic abilities. A good student. He didn't brag about it."
The family has belonged to the church for about 10 years, Borgie said. The suspect's mother, Arlene, attends services every week and volunteers her time.
During the attack early Friday, Holmes set off gas canisters and used the military-style semiautomatic rifle, a shotgun and a pistol to open fire on the unsuspecting theater-goers, Oates said. Holmes had bought the weapons at local gun stores in the past two months. He recently purchased 6,000 rounds of ammunition over the Internet, the chief said.
The gunman's semiautomatic assault rifle jammed during the attack at the Aurora movie theater, forcing him to switch to another gun with less firepower, a federal law enforcement official told The Associated Press. That malfunction and weapons switch during the shooting rampage might have saved some lives.
Oates said a 100-round ammunition drum was found in the theater but said he did not know whether it jammed or emptied.
Police have finished collecting evidence from the apartment where the Colorado shooting suspect lived, but residents are still not allowed back into the building because of chemical hazards. Aurora police said Sunday residents can retrieve personal items, but the building remains closed.
The shooting was the worst in the U.S. since the Nov. 5, 2009, attack at Fort Hood, Texas. An Army psychiatrist was charged with killing 13 soldiers and civilians and wounding more than two dozen others.
Across the street from the movie theater, a man who placed 15 crosses near Columbine High School after a 1999 massacre there has returned to Colorado with 12 crosses for the victims of Friday's shooting.
Greg Zanis, of Aurora, Ill., put up the 3 1/2-foot-tall crosses Sunday on a hill across the street from the Century 16 theater.
Associated Press contributors to this report include Kristen Wyatt, Thomas Peipert and P. Solomon Banda in Aurora; Dan Elliott, Colleen Slevin in Denver; and Alicia A. Caldwell, Eileen Sullivan and Julie Pace in Washington