Ahmad Al Aliwi Alissa faces 10 counts of murder and an attempted murder charge stemming from the rampage on Monday at King Soopers grocery store in Boulder, some 28 miles (45 km) northwest of Denver.
Appearing at a hearing in Boulder County court, Alissa affirmed that he understood his rights under the law and understood that he would be held without bail, as ordered by Judge Thomas Mulvahill.
Prosecutors may file additional charges against Alissa in the coming weeks, District Attorney Michael Dougherty told the judge.
Defense lawyers for Alissa requested that the suspect undergo a full mental health assessment, which would likely push back his preliminary court hearing by a couple of months. Alissa waived his right to a preliminary hearing within 35 days to allow time for that assessment.
Another public defender representing Alissa, Daniel King, defended the gunman who killed 12 people in a movie theater in Aurora, Colorado, in 2012.
The bloodshed at King Soopers was the nation's second mass shooting in less than a week, after a gunman fatally shot eight people at three Atlanta-area day spas on March 16.