James Holmes, 24, is accused of killing 12 people and injuring 58 others as they watched the new Batman movie inside a theater last Friday.
Barely making eye contact with anyone, Holmes showed little emotion in court and had nothing to say to the judge. He had reddish-orange hair, similar to what Heath Ledger's version of the Joker looked like at one point in "The Dark Knight."
One of the victims' families said Holmes had a crazy look in his eye, adding, "Something is seriously wrong with that guy."
See an outline of events and facts surrounding the Colorado theater shooting
The court hearing was not a formal arraignment, like what is usually seen in Southern California. It was an advisement to inform the suspect why he is being held. The judge advised him that there's probable cause he could face first-degree murder charges.
He will be formally charged next Monday. The district attorney said a decision on whether to pursue the death penalty could take much longer.
"I would say there's no such thing as a slam-dunk case. It is a case where we're still looking at the enormous amount of evidence, and we would never presume that it would be slam dunk," said District Attorney Carol Chambers.
Holmes is being held in solitary confinement at a Denver county jail, where he is also under suicide watch. When asked if he was medicated, Chambers said she had no information about that.
Read statements from public officials about the Colorado theater shooting
The Holmes family spoke out for the first time, reiterating through lawyer and spokesman Lisa Damiani that their hearts go out to the victims and their families. Asked if they stand by Holmes, Damiani responded, "Yes they do. He's their son."
Police said Holmes set off gas canisters and opened fire in a packed auditorium minutes into a midnight premiere of the "The Dark Knight Rises" at Century 16 Movie Theaters at the Aurora Town Center.
So far, law enforcement officials said Holmes has not been cooperating with them, and it could take months to learn what prompted the theater attack. Due to a gag order, the Aurora Police Department announced it would no longer be providing updates, comments or interviews on the shooting investigation.
Investigators are hoping to learn more from the items they found inside Holmes' booby-trapped apartment, which was rigged with dozens of bombs. In addition to a "Batman" poster and mask, investigators also removed a computer.
Police said Holmes began buying guns at Denver-area stores nearly two months before Friday's shooting and received at least 50 packages in four months at his home and at school. A gun-range owner near Denver said he recently rejected a membership application from Holmes in part because of a "bizarre" voicemail greeting on Holmes' phone.
Holmes earned a prestigious grant from the National Institute of Health - $26,000 to study neuroscience. But after one year at the University of Colorado Denver, it all ended abruptly. Lilly Marks, executive vice chancellor, said the school has confirmed that there were no dangerous chemicals missing from the labs where Holmes worked.
"He did not fill in his reason for leaving the program," said Barry Shur, dean of the University of Colorado graduate school.
Read more information about shooting suspect James Holmes
As the investigation continues, details about Holmes' background as a student and would-be scientist are trickling out. In video obtained exclusively by ABC News, Holmes is shown giving a lecture at a science camp held at Miramar College in San Diego six years ago. The then-18-year-old Holmes talks about "temporal illusions," the science behind changing your past.
There has already been talk of what the defense's strategy may be and whether they might try and claim insanity."The insanity defense is really tough. You have to demonstrate you didn't understand what you were doing was wrong. And just look at the basics of what we know right now. He was claiming to be the Joker. The Joker is a bad guy, who is doing wrong and doing harm, and that in and of itself, is going to be a real problem," said Dan Abrams, ABC News' legal analyst.
Meantime, several of the 58 people who survived the attack are still hospitalized. Some of them are in critical condition and could still die from their injuries, officials said.
People across the country are coming together to remember the victims of the tragedy. Several thousand people gathered in Aurora Sunday night and prayed for the people killed or injured inside the crowded movie theater. They formed a human chain in solidarity, released purple balloons into the air, and cheered police who responded to the shooting. Colorado Gov. John Hickenlooper read the names of the 12 people who died. After each name, the crowd shouted, "We will remember."
View photos from the scene of the shooting at an Aurora, Colorado theater
