James Holmes, 24, with his hair dyed red and yellow, also was ordered not to have contact with any of the victims or victims' families, and was appointed a public defender in his first court appearance since Friday's shooting during the Century 16 Movie complex's midnight screening Friday of "The Dark Knight Rises," the latest movie in the Batman franchise.
While recapping legal action so far for Holmes, Arapahoe County District Judge William Sylvester said the suspect was jailed under a "no bond hold."
Holmes sat emotionless during the proceeding, sometimes looking ahead and at other times looking down.
The judge also reviewed Holmes' constitutional rights.
The charges on which the court found probable cause to hold Holmes included first-degree murder, Arapahoe County District Attorney Carole Chambers told reporters outside the courthouse.
She didn't know how many charges would be filed next week.
"This is a very active investigation," Chambers said. "We're still doing subpoena ... and search warrants. We're still looking at every angle."
The July 30 hearing is when prosecutors "will give the defense attorney the actual charges the defendant will be facing," Chambers said.
Holmes bought his 6,000-round ammunition supply legally and easily over the Internet, police said. The guns were purchased legally at Denver-area stores.
Police told The New York Times Holmes ordered 3,000 rounds of handgun ammunition, 3,000 rounds for an assault rifle and 350 shells for a 12-gauge shotgun almost as easily as a person orders a book from Amazon.com. He spent an estimated $3,000 at the online sites in the four months before the shooting.
He also bought bulletproof vests and other tactical gear, and a high-capacity "drum magazine" ammunition storage and feeding device that could hold 100 rounds and could fire 50 or 60 rounds a minute, police said.
Holmes, a University of Colorado Denver graduate student in neuroscience with a clean criminal record, bought the ammunition without police awareness because sellers generally aren't required to report sales to law enforcement officials.
Neither Colorado nor federal law required him to submit to a background check or register his purchases, gun policy experts said.
Thousands gathered Sunday evening on the lawn of Aurora's Municipal Building to remember those killed in the shooting.
The vigil followed a visit by President Barack Obama, who told reporters he visited the injured and victims' families "not so much as president as I do as a father and as a husband."
"In the end, after he [Holmes] has felt the full force of our justice system, what will be remembered are the good people who were impacted by this tragedy," Obama said.
In the vigil, Colorado Gov. John Hickenlooper received applause when he said he would not say the suspect's name.
"In my house, we're just going to call him 'Suspect A'," Hickenlooper said.
Hickenlooper then read the names of all 12 victims, asking the crowd to repeat "We will remember" after each name.
Aurora Mayor Steve Hogan asked the audience to acknowledge those in the theater "who went out of their way to help other people."
"Because of them, more lives weren't lost," he said.
Notable deaths of 2014 [PHOTOS]