Holmes is charged with more than 160 counts including murder and attempted murder.
Investigators say he was wearing body armor and a gas mask when he tossed two gas canisters and then opened fire in a theater in the Denver suburb of Aurora on July 20. A midnight showing of the Batman movie "The Dark Knight Rises" had just begun.
In addition to the 12 dead, 70 were wounded.
Many of the survivors and family members of the dead are expected to attend the preliminary hearing, and court officials expect an overflow crowd of reporters and spectators.
The preliminary hearing that starts Monday is expected to last all week. The official purpose is to allow the judge to determine whether the prosecution's case is strong enough to warrant a trial, but legal analysts say it's rare for a judge not to order a trial if a case gets this far.
For Holmes, the hearing could set the stage for a negotiated plea agreement by allowing each side to assess the other's strength, the analysts say.
The hearing will be 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 filed under seal.
It took this long to get to the preliminary hearing because lawyers have been debating what physical evidence should be made available
on 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, started stockpiling weapons, ammunition, explosives and body armor in the spring of 2011. 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 2011, authorities have said.
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The shootings came 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 witnesses who could testify about Holmes' mental health. Prosecutors asked Sylvester to block the witnesses, but he refused.
Press; By DAN ELLIOTT]
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