(RJ Sangosti/Denver Post via AP Photo)

By CLAYTON SANDELL and CAROL McKINLEY:

The man accused of a murderous rampage inside a crowded Aurora, Colo., movie theater last summer has signaled he will pursue an insanity defense.

"Mr. Holmes intends to tender a plea of not guilty by reason of insanity," said attorneys for James Holmes in a court document filed today.

Holmes is accused of opening fire last July 20 during a midnight screening of "The Dark Knight Rises" Batman movie, killing 12 people and injuring another 58 moviegoers. He faces a total of 166 counts, including murder and attempted murder.

Holmes is expected to enter the new plea at a hearing scheduled for May 13, the court document said. A judge had previously entered a "not guilty" plea on Holmes' behalf.

Late Tuesday, the judge overseeing the case filed an order saying that Holmes and his legal team will have to establish "good cause" to permit an insanity plea now that the arraignment has passed.

"After the Court determines whether 'good cause' exists, it will decide how to proceed," Judge Carlos Samour wrote.

Former Adams County, Colo., District Attorney Bob Grant said he believes the judge will allow Holmes to change his plea.

"He won't disallow the entrance of the plea," Grant said.

In April, prosecutors announced they would seek the death penalty.

The decision to pursue an insanity defense was expected. Defense attorneys have repeatedly said in court hearings that they believe Holmes is mentally ill.

Once Holmes enters a plea of not guilty by reason of insanity, he will have to undergo a mental health evaluation conducted by Colorado state psychiatrists.

How that will affect the start of trial, scheduled for February 2014, is not clear.

"The process may take a long time," said Karen Steinhauser, a former Colorado prosecutor turned defense attorney.

An insanity plea also means the confidentiality privilege between a University of Colorado psychiatrist who was treating Holmes before the shooting will be waived.

That would give prosecutors access to whatever Holmes wrote in a notebook he mailed to Dr. Lynne Fenton that was intercepted by police.

Information in the notebook "may assist in determining what methods of planning were involved to carry out this crime," Aurora Police Sgt. Matthew Fyles wrote in an affidavit.

In September, prosecutors dropped their fight for the notebook, figuring they would get access to it anyway once Holmes decided to pursue an insanity defense.

Families of the victims and people injured in the shooting who have observed Holmes in court say they do not believe he is insane.

"He planned it out, disguised himself and planted bombs which were waiting for the police," former prosecutor Grant told ABC News. "He knew what he was doing."

Denver defense attorney Larry Pozner said he believes Holmes will eventually face trial.

"I think we're about to waste millions of dollars of taxpayer money," Pozner said. "He offered to plead guilty and go to jail for the rest of his life. Does anybody think he was in his right mind that night?"