Accused Aurora Theater Shooter James Holmes to Hear Evidence in Preliminary Hearing

This is the most important court hearing in the case so far, essentially a mini-trial as prosecutors present witness testimony and evidence--some never before heard--to outline their case against the former neuroscience student.

The hearing at the Arapahoe County District Court in Centennial, Colo., could last all week. At the end, Judge William Sylvester will decide whether the case will go to trial.

Click here for full coverage of the Aurora movie theater shooting.

Prosecutors say they will present potentially gruesome photos and videos in addition to 911 calls from the night of the shooting that left 12 people dead and 58 wounded. They will aim to convince the judge that there is enough evidence against Holmes to proceed to a trial.

It is expected that the prosecution's witnesses will include the Aurora police lead detective, first responders, the coroner and a computer forensic specialist.

A court-imposed gag order days after the shooting has kept many of the details under wraps, so much of the information could be new to the public.

Hundreds of family members and victims are expected to attend the hearing.

Holmes allegedly opened fire at the crowded Aurora movie theater during a midnight showing of "The Dark Knight Rises" on July 20, 2012. He was wearing a bullet-proof vest, dyed his hair red and was wearing a gas mask.

Holmes has been charged with 166 counts of murder, attempted murder, possession of explosives and crime of violence. The district attorney has not decided whether to seek the death penalty, and Holmes' defense team believes Holmes is mentally ill. He has not entered a plea.

One of the attendees will be MaryEllen Hanson, whose great-niece Veronica Moser Sullivan, 6, was killed in the shooting. Veronica's mother Ashley was shot and is now a quadriplegic and suffered a miscarriage.

"It's one of those things that you almost have to face the devil," Hanson told ABC News. "I don't feel he has the right to intimidate people. I think it's really important to know the details."

Hanson said she will have to "brace herself" to see and hear photos and videos, but is firm in her desire to have a "first-hand experience" with the proceedings.

"I want to know the facts," she said. "There's been a lot of misinformation out there and a lot of information that hasn't been revealed...I need to know what happened that night so I have a better understanding of the horror." Hanson said that she sees Holmes as a "very troubled person."

"The first time I was in court to see him...I felt he was a personification of evil, extremely troubled," she said. "I just can't wrap my head around how someone can be like that and do the things he's done."

She hopes that an understanding of what happened can provide some closure, even though she doesn't foresee ever fully healing from what happened.

"I hope to get to a place where we can move forward," she said. "I really don't think that James Holmes should leave a large footprint in the community."