DENVER (Reuters) - Attorneys for 20 news organizations will be in a Colorado courtroom on Thursday to argue that documents detailing the murder case against accused movie theater gunman James Holmes should be unsealed by a judge and open to the public.

A Colorado judge ordered most court documents sealed at the request of prosecutors shortly after the arrest of Holmes, who is accused of opening fire at a midnight screening of the new "Batman" movie in the Denver suburb of Aurora in July.

Twelve people died and 58 others were wounded in the shooting rampage. Last week, prosecutors charged Holmes, a 24-year-old former neuroscience graduate student at the University of Colorado, with 24 counts of first-degree murder and 116 counts of attempted murder.

Media lawyer Steven Zansberg said in a written motion that the decision to close the bulk of the file "undermines our nation's firm commitment to the transparency and public accountability of the criminal justice system."

In the motion, lawyers for the media outlets, including The New York Times Co., The Associated Press, The Denver Post, and CBS News, argued that the U.S. Constitution and state case law requires a presumption of openness. Thomson Reuters is not a party to the motion.

"Absent disclosure of the factual basis for the issuance of a warrant, the public cannot properly assess the propriety of the government's conduct," the motion said.

In anticipation that prosecutors might argue that opening the file might compromise the criminal probe, the motion states that authorities have already said publicly that Holmes acted alone, so there is no ongoing investigation of other suspects.

Citing a separate "gag" order prohibiting involved parties from making public statements about the case, a spokeswoman for District Attorney Carol Chambers declined to comment.

The same unknowns hold true for any answer by Holmes' lawyers, but in a previous court hearing public defender Tamara Brady said it was her position that the file should remain sealed.

The media motion said that in other high-profile Colorado cases opening the court file did not hurt a defendant's due process rights, citing the dismissal of sexual assault charges lodged against Los Angeles Lakers superstar Kobe Bryant.

More recently, the motion said, investigative files in the rape case against a former Denver Bronco football player were opened, and the defendant was acquitted of the charges after a jury trial.

The Colorado state public defender's office said they had no comment about the case.

There are other options available to the court to ensure Holmes receives a fair trial, the motion said.

Those include expanding the number of people in the jury pool, allowing extensive questioning of potential jurors by the lawyers, and seeking a change of venue if pretrial publicity is too pervasive.

Holmes is being held without bond and in solitary confinement at the Arapahoe County jail, and it was unclear whether he will be in court for Thursday's hearing.