A US judge on Monday lifted a gag order on documents of the case against the Batman movie killing in Colorado, giving the public a closer look at one of the worst mass shootings in US history.

Chief Judge William Sylvester decided to unseal 34 court documents about the legal proceedings of the case, including a motion for access and preservation of the crime scene, and relative evidences.

After James Holmes, the suspected killer, was arrested on July 20, the judge banned the media's access to these papers, saying if disclosed, they would disturb the ongoing investigation and violate the defendant's right to be fairly judged.

The disclosure is "a victory for the people", said attorney Steven D. Zansberg, who represents 21 media outlets.

"The decision is consistent with how the court system operates in our country, and the public should not be kept in the dark about this legal proceeding," Zansberg said.

Despite many files were unsealed, Judge Sylvester decided to keep confidential Holmes' information in the University of Colorado, where he was a former neurology PhD candidate.

Holmes, 24, was arrested outside a theater after allegedly killing 12 people and wounding another 58 at the midnight showing of a Batman movie on July 20. He made the first court appearance three weeks ago.