James Holmes: At Court Hearing, Police Describe Moments After Movie Shooting

Prosecutors began presenting evidence on Monday against James Holmes, accused of 166 counts of murder and attempted murder in connection with shootings last July 20 at a midnight screening of The Dark Knight Rises.

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At an important preliminary hearing, Jason Oviatt, one of the police officers who arrested Holmes, described the defendant as being "relaxed" and "detached" moments after the shooting that killed 12 people and left dozens of others wounded. The officer also says he initially mistook Holmes to be a fellow officer because he was wearing a gas mask, a helmet and body armor, according to various media reports.

The officer then said he realized otherwise after seeing that Holmes wasn't running.

Another officer took the stand and detailed some of the results of the investigation that found that Holmes had purchased his ticket a couple weeks in advance on July 8

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Family of those killed at the shooting along with other victims were present at the hearing. Holmes, fully-bearded and wearing a a red jumpsuit, sat calmly during the testimony, .

The hearing is being held at the Arapahoe County District Court and could last several days as the judge decides whether there is sufficient evidence to put the 25-year-old on trial. Much of the evidence being presented will be shown in public for the first time -- and, up until now, has been under seal with the attorneys being prevented from speaking to the press due to a gag order.

One of the items of interest likely to be detailed are the contents of a notebook that Holmes sent a psychiatrist at the University of Colorado Denver, where he was a neuroscience doctoral student.

Holmes' attorneys are also expected to call witnesses, according to ABC News, including an expert who will testify about his mental condition.