Scott Ostrem, 47, was told during a brief hearing in Adams County District Court in Brighton that he had been charged with six counts of murder and 30 counts of attempted murder.
The six murder counts include two for each slain victim, under different legal theories.
No one else was wounded in the attack, but prosecutors said the attempted murder charges referred to other people in the store who could have been struck by gunfire.
The charges could make Ostrem eligible for the death penalty if he is convicted. Adams County District Attorney Dave Young told reporters outside court that he had not decided whether to seek it.
"The (victims') families will certainly be a part of that determination," Young said, adding that additional charges could be filed in the case.
The defendant, who was shackled and dressed in yellow and white jail garb, gave one-word answers to the judge. He did not enter a plea.
Police said they had yet to establish a motive for the rampage last Wednesday, which took place amid a string of US mass shootings that have renewed calls for restrictions on gun ownership.
Early accounts of multiple casualties also revived painful memories for the Denver area.
In 2012, a gunman killed 12 people at a midnight screening of the Batman movie "The Dark Knight Rises" at a theater in the suburb of Aurora. The shooter, James Holmes, is serving a dozen consecutive life sentences without the possibility of parole.
In 1999, two high school seniors fatally shot 12 fellow students and a teacher at Columbine High School in suburban Jefferson County. The pair, Eric Harris and Dylan Klebold, then committed suicide in the campus library.