The man suspected of killing 12 people at a screening of the new Batman movie last month in Colorado has been described as "mentally ill" by his defence lawyer at a court hearing.

James Holmes, a psychiatric patient, faces two counts of first degree murder for each person killed in the July 20 massacre in Aurora, Colorado - one for each victim's deliberate killing and one more for killing through "depraved indifference" to life.

But at a hearing on Thursday, defence attorney Dan King chastised prosecutors for not turning over all the reports, photos from the crime scene and other evidence.

"We cannot begin to assess Mr Holmes's mental illness until we have all of that information," said King, who referred to his client's mental illness two other times.

Holmes, 24, until recently a neuroscience student, allegedly burst into a midnight viewing of "The Dark Knight Rises" and claimed he was "The Joker" - Batman's sworn enemy in the comic book series, before opening fire.

Ten of the 58 people who were wounded in the attack remain hospitalised and four are in a critical condition, leaving open the possibility that the huge charge sheet could yet grow longer.

At Thursday's hearing, attorneys discussed the merits of revising the judge's order barring public access to court records, following media requests.

Prosecutors argued that sealing the court documents protects the integrity of the ongoing investigation - which so far has generated 2,677 pages of evidence - and defence lawyer King said continuing to suppress access to the records protects his client's right to a fair trial, but no decision was made.

Judicial District Court Chief Judge William Sylvester did not indicate when he would rule on the media request but he set the next scheduled court hearing for August 16.

That hearing will discuss whether a package the former graduate student allegedly sent to a University of Colorado psychiatrist should be considered protected communication under doctor-patient confidentiality privilege.

Holmes also faces 116 attempted murder charges for wounding 58 people in the theater with indiscriminate firing, one count of possession of an explosive device and one sentence enhancement count for a "crime of violence."

Prosecutors have said it will be several weeks before a decision is made on whether or not to seek the death penalty for Holmes. Only one person has been executed in Colorado since capital punishment was reinstated in 1976.

Source: Agencies