The man, Ahmad Al Aliwi Alissa, 21, wore a purple smock and a white mask and he sat in a wheelchair during a hearing before state District Judge Thomas Mulvahill, nearly three days after the massacre at a King Soopers grocery store.
"Our position is that we cannot do anything until we are able to fully assess Mr. Alissa's mental illness," defense attorney Kathryn Herold told the court.
Boulder County District Attorney Michael Dougherty said during the brief hearing that more charges are likely within the next two weeks.
Dougherty said that investigators were still working the crime scene Thursday and that his office has not received any police reports that he could share with the defense.
Mulvahill ordered Alissa held without bail and said his next hearing is to be no sooner than in 60 days.
Colorado law allows for a mental illness defense, but it's a highly unlikely path to acquittal, said Martin Stuart, chairman of the Colorado Bar Association's Criminal Law Section.
He cited the case of the killer at a movie theater in Aurora in 2012, who had documented mental health issues but was still convicted of all murder charges against him.
"It's very rare. It's rare that someone pleads not guilty by reason of insanity, and it's very rare" that it works, Stuart said Thursday. "It is a very difficult defense to prevail upon."
Sara Hildebrand, a clinical teaching fellow at the University of Denver's Sturm College of Law, said Alissa's mental health profile could play a role in determining when he might be deemed fit to face a jury.
"Some people are deemed incompetent to stand trial due to mental health-related challenges and diagnoses and must undergo attempts at restoring them to competency before most proceedings in their criminal case can go on," Hildebrand said.
The last time the public caught a glimpse of Alissa was on Monday afternoon outside the market, when an aerial news video showed police escorting a man in handcuffs, his right leg covered in blood. He wasn't wearing a shirt or shoes.
While authorities have declined to definitively link the video to Alissa, an arrest affidavit described how the suspect surrendered to a SWAT team. The description is consistent with what was in the video.
Alissa is also accused of killing Denny Stong, 20; Neven Stanisic, 23; Rikki Olds, 25; Tralona Bartkowiak, 49; Suzanne Fountain, 59; Teri Leiker, 51; Kevin Mahoney, 61; Lynn Murray, 62; and Jody Waters, 65.
Colorado residents have become familiar with mass shootings, as two of this generation's most notorious massacres took place within short drives from Boulder.
And it is 35 miles northwest of the Century 16 Theater in Aurora, where a gunman opened fire on moviegoers on July 20, 2012. Twelve people, who had gathered for a screening of "The Dark Knight Rises," were killed.