"He seemed very detached from it all," he said.
When Oviatt first saw Holmes in his gear standing next to his car behind the theater, he thought he was a fellow officer but then realized Holmes was standing still, and not rushing toward the theater.
Oviatt pointed his gun at him, handcuffed him and searched him. He said he found two knives and a semi-automatic handgun on top of Holmes' car. Oviatt said an ammunition magazine also fell out of Holmes' pocket and he found another one on the ground. He said Holmes was dripping in sweat and his pupils were wide open.
Officer Aaron Blue said Holmes was fidgeting around after he and Oviatt put him in a patrol car, prompting them to stop and search Holmes again. They were worried they might have missed something because of Holmes' bulky outfit.
Investigators say Holmes tossed two gas canisters and then opened fire during the midnight showing of the Batman movie "The Dark Knight Rises" on July 20, killing 12 people and wounding dozens.
The preliminary hearing is expected to last all week. It will allow the judge to determine whether the prosecution's case is strong enough to warrant a trial but it's rare for a judge not to order a trial if a case gets this far.
With some families of shooting victims listening, police officers fought to keep their composure as they testified about their efforts to try to save the wounded without enough ambulances.
The movie was still playing as they entered the theater. An alarm was going off and moviegoers' cellphones rang unanswered. There was so much blood on the floor, officer Justin Grizzle said he slipped and almost fell down.
As Sgt. Gerald Jonsgaard recalled not finding a pulse on the youngest victim, 6-year-old Veronica Moser-Sullivan, a woman in the courtroom sat with her head buried in her hands.
Grizzle recalled ferrying the wounded to the hospital and said he had to stop one man worried about his 7-year-old daughter from jumping out of the moving patrol car. He said there was so much blood in his car that he could hear it sloshing around.
A bearded Holmes didn't appear to show any emotion.
Whatever details emerge at the preliminary hearing, they will do so in a nation that has changed dramatically since the July 20 attack that pushed the problems of gun violence and mental illness into the forefront before receding.
That debate reignited last month when a gunman killed 20 children and six adults at Sandy Hook elementary school in Newtown, Conn., spawning calls for better psychiatric care, tougher gun laws and the arming of teachers.
Holmes is charged with more than 160 counts, including murder and attempted murder.
Legal analysts say that evidence appears to be so strong that Holmes may well accept a plea agreement before trial.
In such cases, the preliminary hearing can set the stage for a deal by letting each side assess the other's strengths and weaknesses, said Laurie Levenson, a former federal prosecutor and now a professor at Loyola Law School in Los Angeles.
They "are often the first step to resolving the case, a mini-trial so both sides can see the writing on the wall," she said.
In general, plea agreements help prosecutors avoid costly trials, give the accused a lesser sentence like life in prison rather than the death penalty, and spare the victims and their families from the trauma of going through a lengthy trial.
At this stage, prosecutors must only meet a "probable cause" standard much lower than the "beyond a reasonable doubt" standard for a guilty verdict, said Mimi Wesson, a professor of law at the University of Colorado Law School.
Many of the survivors and family members of the dead attended the hearing.
The hearing is the first extensive public disclosure of the evidence against Holmes. Three days after the shooting, District Judge William Sylvester forbade attorneys and investigators from discussing the case publicly, and many court documents have been under seal.
It took this long to get to the preliminary hearing because lawyers have been debating what physical evidence should be made available to one side or the other, whether a psychiatrist who met with Holmes is barred from testimony by doctor-patient privilege, who was responsible for leaks to the media, and other issues.
Police say Holmes, now 25, had stockpiled weapons, ammunition, explosives and body armor. He was a first-year student in a Ph.D. neuroscience program at the University of Colorado, Denver, but he failed a year-end exam and withdrew in June, authorities have said.
The shootings happened six weeks later.
Federal authorities have said Holmes entered the theater with a ticket and is believed to have propped open a door, slipped out to his car and returned with his weapons. Police arrested him outside the theater shortly after the shootings ended.
Holmes' mental health could be a significant issue and possibly a contentious one in the preliminary hearing.
His attorneys have told the judge Holmes is mentally ill, but they have not said whether they plan to employ an insanity defense. He had seen a university psychiatrist, and his lawyers have said he tried to call the psychiatrist nine minutes before the killing began.
Defense lawyers have said they plan to call at least two people, described as "lay witnesses," who could testify about Holmes' mental health. Prosecutors asked Sylvester to block the witnesses, but he refused.