James Holmes' lawyers made the disclosure at a court hearing in Centennial, Colo., where news media organizations were asking a judge to unseal documents in the case.
Holmes had the same dazed demeanor that he has had in previous court appearances.
Defense attorney Daniel King made the revelation as he argued defense attorneys need more information from prosecutors and investigators.
"We cannot begin to assess the nature and the depth of Mr. Holmes' mental illness until we receive full disclosure of all the documents," King said.
News media organizations were asking the judge to unseal court documents and scale back a gag order that bars the university that Holmes attended from releasing details about him.
The Associated Press and 20 other news organizations want Chief District Judge William Sylvester to unseal documents that could provide the public with details about Holmes and the massacre in Aurora on July 20.
The shooting during a midnight showing of the latest Batman movie left 12 people dead and injured 58 others. Holmes, a 24-year-old former Ph.D. student at the University of Colorado, is charged with multiple counts of first-degree murder and attempted first-degree murder.
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 defense 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 at a midnight showing of "The Dark Knight Rises," 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 Holmes' apartment, 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.
In Colorado, this type of legal battle has been seen before.
In 2007, an Arapahoe County judge sealed an indictment in the case of a missing 6-year-old girl whom authorities determined had been dead for at least two years before her father, Aaron Thompson, reported her missing. The state Supreme Court ordered the indictment unsealed in 2008, allowing the public to learn the charges against Thompson. Thompson was convicted of fatal child abuse in 2009.
When Los Angeles Lakers player Kobe Bryant faced sexual assault charges in Vail in 2003, it took a media challenge to unseal an affidavit in which police laid out their case for an arrest. Bryant maintained his innocence, and prosecutors dropped the case in 2005.
A news media challenge led to last year's release of an arrest affidavit in a sexual assault case involving former Denver Broncos cornerback Perrish Cox. Cox was acquitted in March.
Defense attorneys and prosecutors routinely ask judges to keep some documents sealed, often because the documents contain information a jury won't hear at trial, said Denver criminal defense attorney Daniel Recht, who also argues First Amendment cases.
But Moore noted that some Colorado judges have sealed entire court dockets under the argument that the mere fact of media coverage will damage a case.
In his ruling to unseal documents in the Cox case, Douglas County District Judge Paul A. King rejected that notion.
"There can be no presumption that everyone in the jury panel will read, follow and find important the media accounts in this case," King wrote.