CENTENNIAL, Colo. -- Attorneys for the suspect in the Colorado movie theatre shootings said Thursday their client is mentally ill and that they need more time to assess the nature of his illness.

James Holmes' lawyers made the disclosure at a court hearing in suburban Denver where news media organizations were asking a judge to unseal court documents in the case.

Holmes, a 24-year-old former Ph.D. student at the University of Colorado, Denver, had the familiar, dazed demeanour that he has had in previous court appearances.

Holmes is accused of going on a July 20 shooting rampage at a midnight showing of the latest Batman movie in Aurora, killing 12 people and injuring 58 others.

Defence attorney Daniel King made the revelation about Holmes as he argued defence attorneys need more information from prosecutors and investigators to assess their client.

"We cannot begin to assess the nature and the depth of Mr. Holmes' mental illness until we receive full disclosure," he said.

King said Holmes sought out university psychiatrist, Lynne Fenton, for help. He did not elaborate. A hearing was scheduled for Aug. 16 to establish whether there is a doctor-patient relationship between Fenton and Holmes.

Twenty-one news organizations, including The Associated Press, were also asking Chief District Judge William Sylvester to scale back a gag order that bars the university from releasing details about Holmes.

Arapahoe County prosecutors argue releasing documents could jeopardize their investigation. Holmes' attorneys want to ensure he receives a fair trial.

Sylvester's order sealing documents includes the case file, which makes it impossible for observers to understand prosecution and defence arguments on motions that are referenced by number only.

Sylvester on July 23 also issued a gag order that bars officials at the University of Colorado from responding to public records requests concerning Holmes.

The judge said doing so would jeopardize the county's investigation. Aurora officials have cited the order in declining to speak about the city's response to the shootings.

"It is performing our watchdog role to look at the process and try to assess for the public how the police have handled the case and assembled the evidence and assure for the defendant and the public that things are being conducted open and fairly," said Gregory Moore, editor of The Denver Post. "It goes way beyond what's necessary to protect the defendant's right to a fair trial."

Court documents, which include search warrants, inventories of evidence collected by police and police interviews with witnesses can be an important source of information for the public.

Little is known about how police say Holmes prepared for the shooting, or how they say he rigged his nearby apartment with explosives. Aurora Police Chief Daniel Oates has said the explosives were designed to kill anybody who entered, including first-responders.

Steven D. Zansberg, an attorney representing the news media consortium, said the judge should at least explain which documents have been sealed and why.