DENVER (Reuters) - A Colorado judge on Monday ordered that the bulk of key documents detailing the case against accused movie theater gunman James Holmes remain sealed, largely rejecting a request by news organizations to reverse a previous order.

Arapahoe County District Judge William Sylvester did allow for the release of 34 documents, mostly relating to media motions and procedural issues.

But he ordered that the documents media members most wanted to see - affidavits of probable cause, subpoenas, arrest warrants, search warrants, and requests for or court orders for production of records - all be kept under seal.

Prosecutors have charged Holmes with 24 counts of first degree murder and 116 counts of attempted murder in the July shootings at a midnight screening of the Batman film "The Dark Knight Rises" in Aurora, Colorado. Twelve people were killed and 58 were wounded.

Holmes, a 24-year-old former graduate student, was arrested shortly after the massacre. Authorities said he told police he was the "Joker" in a reference to Batman's comic-book nemesis.

He is being held in solitary confinement at the Arapahoe County jail.

Lawyers for 20 news organizations, including The New York Times, The Associated Press, The Denver Post and CBS News last week asked Sylvester to unseal the documents, arguing that such secrecy in a high-profile case "undermines our nation's firm commitment to the transparency and public accountability of the criminal justice system."

Thomson Reuters is not a party to the motion.

Media attorney Steven Zansberg, said the judge's limited ruling "brought much needed transparency" to the case but did not go far enough.

'FAR-REACHING CONSEQUENCES'

"We are disappointed that the affidavits of probable cause remain under seal at this time, but are hopeful that the court will revisit that issue sometime in the not too distant future," he said in a statement.

Prosecutors and defense lawyers have both objected to unsealing the documents, saying it was too early to open the file because the investigation was still ongoing. In most court cases, documents are open to the public.

Public defender Daniel King said at a court hearing last week that Holmes, a former neuroscience graduate student at the University of Colorado, suffered from a "mental illness" and had sought help before the mass shooting from university psychiatrist Lynne Fenton.

The remark was seen by legal experts as a signal that Holmes' defense attorneys may seek to mount an insanity defense on his behalf.

In Monday's ruling, the judge repeated his earlier order preventing the university from publicly discussing its dealings with Holmes because release of that information might violate therapist-patient privilege.

If privileged communications were disclosed it "would have serious far-reaching and potentially irreparable consequences to this case," the judge wrote.

A package that Holmes reportedly sent to Fenton is in the possession of the court clerk and will remain sealed and "inaccessible to anyone," he said.

Whether the package falls under the therapist-patient privilege will be addressed at a hearing later this week.

As in many states, mental healthcare providers in Colorado must warn authorities of potential violent behavior only when a patient has communicated a serious threat of imminent physical violence against a specific target.