A judge ruled late Thursday that there's enough evidence for James Holmes to face trial on charges that he killed 12 people and injured 70 others in a Colorado movie theater last summer.

Judge William Sylvester said prosecutors have established probable cause to proceed with 166 felony counts, including murder and attempted murder.

Holmes is due to be arraigned Friday, but his defense attorneys filed papers Thursday afternoon saying he's not ready to enter a plea. They are likely to appear in court Friday to ask for the arraignment to be delayed.

Defense attorneys did not explain why they are not ready for arraignment. Their filing also objected to media requests to bring cameras into the courtroom. Other than during his brief initial appearance in July, cameras have been barred from court during Holmes' case.

Sylvester's ruling came after a three-day hearing earlier this week, in which prosecutors laid out their case against Holmes, 25.

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A succession of police and federal agents testified that Holmes spent weeks amassing guns and ammunition, concocted explosives to booby-trap his apartment and scouted the movie theater where he would allegedly unleash a horrific attack on hundreds of terrified people.

The officers also described a hellish scene inside the theater on July 20, when 12 people were shot to death before their families and friends' eyes and scores of others were wounded amid a din of gunshots, screams and the blaring soundtrack of "The Dark Knight Rises."

Holmes' lawyers called no witnesses and cross-examined only a few of those summoned by prosecutors during the hearing. But they pointedly raised the issue of Holmes' sanity at strategic moments, possibly foreshadowing a defense that some believe is his best hope to avoid the death penalty.

"You're aware that people can be found not guilty on the grounds of insanity?" defense attorney Daniel King asked one witness.

The preliminary hearing, which ended Wednesday, was designed to determine whether prosecutors' case is strong enough to put Holmes on trial.

Holmes' lawyers haven't said if he will plead not guilty by reason of insanity, but since his arrest outside the theater in the Denver suburb of Aurora immediately after the shootings, they have portrayed him as a man with serious mental problems prone to bizarre behavior.

Many legal analysts have said they expect the case to end with a plea bargain rather than a trial.

Tom Teves, whose son Alex was among the dead, said he would rather see Holmes plead guilty to first-degree murder, avoiding a traumatic trial, bringing a life sentence and closing the door to an insanity defense.

If found not guilty by reason of insanity, Holmes could conceivably be released someday if he is deemed to have recovered.

"Don't pretend he's crazy," Teves said Wednesday. "He's not crazy. He's no more crazy than you and I."

Prosecutors developed twin themes at the hearing: the horror and devastation of the attack, and a weekslong process in which they alleged Holmes planned and prepared for the assault.

Two officers were overcome by emotion when they testified about the chaos in the theater and the race to get victims to hospitals by police cars until ambulances could arrive. Other testimony included the names and injuries of the victims, read out one by one.

Prosecution witnesses also testified that Holmes started assembling an arsenal in early May and by July 6 had two semi-automatic pistols, a shotgun, a semi-automatic rifle, 6,200 rounds of ammunition and high-capacity magazines that allow a shooter to fire more rounds without stopping to reload.

In late June he began equipping himself with a helmet, gas mask and body armor, the witnesses said.

In early July, they testified, he began buying fuses, gunpowder, chemicals and electronics to booby-trap his apartment in hopes of triggering an explosion and fire to divert police from the theater. The bombs never went off.

Also in early July, he took some interior and exterior photos of the theater, witnesses said.

"He picked the perfect venue for this crime," prosecutor Karen Pearson said.

On Wednesday, Pearson showed a series of photos that investigators said Holmes took of himself hours before the massacre. In one, he glares through black contact lenses, sticking out his tongue, as two locks of his orange-dyed hair curl out on either side of his head like horns.

Caren Teves, mother of Alex and wife of Tom Teves, said she saw Holmes smile when his self-portraits were shown in court.

"He just sat in the courtroom pretty much delighted. He was smiling. He was smirking," she said.