"It seems to me you should be able to narrow that down a lot," Judge Carlos Samour said during a hearing Monday.
James Holmes is charged with killing 12 people and attempting to kill dozens more in the shooting rampage in an Aurora movie theater. Holmes' attorneys have admitted he committed the act but said he did so during a psychotic episode.
Prosecutors are not required to furnish a "good faith" witness list to a defendant's attorneys, but defense attorney Dan King said Holmes' legal team would not be prepared for the scheduled February trial date without a narrower list.
Monday's hearing was the start of at least three weeks of hearings in which decisions about whether large amounts of evidence, including property found in Holmes' car and apartment and information learned from his emails, should be heard during the trial, the Denver Post reported Tuesday.