Centennial, Colorado (CNN) -- So much blood the theater floor had become slippery. Bodies with horrific injuries. The eerie sound of cell phones ringing, over and over again.

This is the scene Aurora police Officer Justin Grizzle said he encountered moments after entering the theater where, according to authorities, 25-year-old James Holmes killed 12 people in a July 20 shooting rampage.

Grizzle testified Monday on the opening day of the preliminary hearing for Holmes, who is charged with 166 counts of murder, attempted murder and weapons charges.

The 13-year veteran wiped away tears while describing his efforts to rush badly wounded victims to the hospital in his police cruiser, including shooting victim Ashley Moser and her husband, who wanted Grizzle to turn around and head back to the theater.

"He was shot in the head somewhere. He kept asking where his ... daughter was," Grizzle said. "He opened the door and tried to jump out."

Grizzle said he had to drive and hold the man by his shoulder to keep him in the car.

The girl the man was seeking, 6-year-old Veronica Moser-Sullivan, was among those killed in the shooting at a midnight showing of "Batman: The Dark Knight Rises." Veronica's mother, Ashley, faces a long recovery after being paralyzed in her lower half and miscarrying after the shooting.

The preliminary hearing that began Monday is designed to show a judge that the state has enough evidence to proceed to trial. Prosecutors are calling scores of witnesses and outlining their evidence in the case. The hearing could go on for days.

A gag order imposed by the judge in the case has limited the flow of information about the attack. However, a source said Holmes allegedly went out a rear exit door, propped it open and gathered his weapons. He then returned to the theater and tossed a canister inside before opening fire, the source said.

Screaming moviegoers scrambled to escape from the gunman, who shot at random as he walked up the theater's steps, according to witnesses.

It was a scene "straight out of a horror film," said Chris Ramos, who was inside the theater.

While none of the four law enforcement witnesses who testified Monday offered insight into a possible motive for the shooting, some new details emerged.

Police Sgt. Gerald Jonsgaard said Holmes stopped the theater door from locking by using a small piece of plastic commonly used to hold tablecloths onto a picnic table. Jonsgaard also said he spotted a a shotgun and a large drum magazine that appeared to be to jammed on the floor of the theater.

Holmes' attorneys are expected to argue that their client has "diminished capacity," a term that, according to the Colorado Bar Association, relates to a person's ability or inability "to make adequately considered decisions" regarding his or her legal representation because of "mental impairment or for some other reason."

After the hearing, Arapahoe County District Judge William Sylvester will determine whether there is enough evidence for Holmes to stand trial.

Security was tight at the hearing. Spectators had to pass through a metal detector and then were searched again before entering the courtroom. At least nine armed officers stood guard inside, some of them scanning the audience packed with reporters and victims' family members.

Holmes appeared expressionless during the hearing and did not speak. His bushy hair and long beard contrasted with the bright red hair and close-cropped looks he sported during previous appearances.

During portions of the hearing, family members of victims held one another, sobbing.

Earlier in Monday's hearing, police Officer Jason Oviatt -- the first officer to encounter Holmes after the rampage ended -- testified that Holmes seemed "very, very relaxed."

Holmes, his pupils dilated, sweating and smelly, didn't struggle or even tense his muscles as he was dragged away to be searched.

"He seemed very detached from it all," Oviatt testified, describing Holmes as unnaturally calm amid the chaos and carnage.

Oviatt testified Monday that within minutes of the first calls, he responded to the theater and found Holmes standing outside in a helmet and gas mask, his hands atop a white coupe that turned out to belong to him.

At first, Oviatt said, he thought Holmes was a police officer, but as he drew within 20 feet, he realized something was terribly wrong.

"He was just standing there. All the other officers were running around, trying to get into the theater," Oviatt said.

A trail of blood led from the theater. The rifle that authorities believe Holmes used in the attack lay on the ground near the building.

Holmes calmly complied with all of Oviatt's orders, the officer testified.

Another officer, Aaron Blue, testified later that Holmes matter-of-factly told him, without prompting, about the complex web of explosives that authorities would later find in his Aurora apartment.

He told Blue that the devices "wouldn't go off unless we set them off."

Holmes was a doctoral student in the neuroscience program at the Anschutz Medical Campus of the University of Colorado, Denver, in Aurora, until he withdrew a month before being arrested outside the bullet-riddled movie theater. He had been a patient of a University of Colorado psychiatrist, according to a court document filed by his lawyers.

His only brush with the law in Colorado appears to have been a 2011 summons for speeding from Aurora police.

If Holmes is ruled incompetent to stand trial, the hearing could provide the best opportunity for victims and the public to understand what happened and why.

To at least one victim, it doesn't matter if Holmes stands trial.

"I obviously don't want him to walk, but as long as he doesn't see the light of day again, it doesn't really much concern me beyond that," said Stephen Barton, who suffered wounds on his face, neck and upper torso in the shooting that night. "To me, I see the trial as being an opportunity to learn more about what happened that night beyond just my own personal recollection."

New Details Emerge In Colorado Theater Shooting