Prosecutors have revealed the first extensive public details of last year's mass shooting at a Colorado cinema, outlining their case against the man suspected of killing 12 people and wounding at least 58.

The hearing on Monday will determine whether the case will go to trial. Legal analysts said evidence appears to be so strong that James Holmes may accept a plea agreement before trial.

Holmes is charged with more than 160 counts in the Colorado attack, including murder and attempted murder.

Investigators say he was wearing body armour and a gas mask when he tossed two gas canisters and opened fire in the cinema on July 20.

On Monday, a police officer who arrested Holmes said he at first thought he was a fellow officer because he was wearing body armour.

Then the officer realised he was wrong because Holmes was not running towards the scene.

Police officers who arrested Holmes described him as unusually relaxed but fidgety at times, that he seemed detached and volunteered that his apartment had been booby trapped.

Until now, many details of the case have been kept quiet. 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.

Police say Holmes, now 25, had stockpiled weapons, ammunition, explosives and body armour.

Mentally ill?

Holmes was a first-year student in a PhD neuroscience programme at the University of Colorado, Denver, but he failed a year-end exam and withdrew, authorities have said. The shootings came six weeks later.

Federal authorities have said Holmes entered the cinema 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 movie theatre shortly after the shootings ended.

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.

Holmes' mental health could be a significant issue in the hearing.

His attorneys have told the judge Holmes is mentally ill, but they have not said whether they plan to employ an insanity defence.

He had seen a university psychiatrist, and his lawyers have said he tried to call the psychiatrist nine minutes before the killing began.

Defence lawyers have said they plan to call at least two witnesses who could testify about Holmes' mental health. Prosecutors asked the judge to block the witnesses, but he refused.

Lawyers have been debating what physical evidence should be made available, whether a psychiatrist who met Holmes is barred from testimony by doctor-patient privilege and other issues.