The man accused of shooting dead 12 people inside a Colorado cinema was relaxed but "out of it" in the moments after the massacre, a court has heard .

James Holmes, who is alleged to have sprayed the audience of a midnight showing of The Dark Knight Rises with bullets, wounding 58 people, appeared unmoved as he "stared into space", the police officer who arrested Holmes told the hearing.

"It was like there weren't normal emotional responses", officer Jason Oviatt said, according to a report of proceedings by the Denver Post. The testimony came as prosecutors in the US began laying out their case against Holmes, watched by family members of those who were killed.

The session, at the Arapahoe county justice centre, will decide if the suspect is sent to trial over the killings. Holmes, 25, is charged with more than 166 separate offences relating to the mass shooting of 20 July in Aurora, including first degree murder.

If convicted, and prosecutors decline to pursue a death penalty sentence, he would face a mandatory sentence of life without parole.

The Arapahoe court is expected to hear the case against Holmes for the first time. Many of the details of the police investigation into the events surrounding the massacre have been kept out of the public domain until now.

In the aftermath of the shooting, district judge William Sylvester issued an order banning lawyers and police from discussing the case with people outside the investigation. Also, many documents have remained secret.

Monday's session opened with testimony from police officers who arrested Holmes outside the cinema. They described him as relaxed but fidgety. Oviatt testified that Holmes was "just standing there … not doing anything. Not in any hurry. Not excited. Not urgent about anything," the Denver Post reported.

Holmes, still clad in body armour, told police he had booby-trapped his apartment. It took officers hours to disarm explosive devices left at Holmes' address with the help of a controlled robot.

After setting the trap at his Aurora apartment, Holmes is alleged to have gone to the cinema and bought a ticket for the late-night screening of The Dark Knight Rises. It is believed that shortly after the film started, he slipped out of a side exit, propping the door open as he left.

He then returned heavily armed and wearing body armour. Some audience members thought he was part of a prank. But bewilderment quickly turned to horror after the gunman tossed two gas canisters into the room and began firing, spraying the audience with bullets. Within minutes, a dozen people were dead and many more were left with gunshot wounds.

Holmes' lawyers have said their client is mentally ill. During the course of this week's hearings, they are expected to call at least two experts to testify on defendant's mental state. He is known to have been visiting a university psychiatrist, whom he had tried to contact shortly before the shootings.

Some legal experts have suggested that the case against Holmes appears so strong that a plea agreement is likely.

As such, this week's preliminary hearing is being treated by some as a 'mini-trial'. Family members of his alleged victims and those injured in the massacre were allowed to sit in a separate room away from the media and public. © Guardian News and Media 2013