The judge in the US Batman cinema massacre case has entered a not guilty plea on behalf of James Holmes after his defence team said he was not ready to enter one.

Holmes, who killed 12 people and injured 70, could be executed or spend the rest of his life in prison if he is convicted.

Judge William Sylvester said Holmes, 25, can change his plea to not guilty by reason of insanity later, if he chooses.

Holmes is charged with multiple counts of murder and attempted murder for the July 20 attack at a suburban Denver cinema during a midnight showing of the latest Batman film The Dark Knight Rises.

The attack was one of a string of public mass killings last year that launched a fierce national debate over gun control.

Prosecutors for their part have not said yet whether they will pursue the death penalty, announcing that they will make their decision known on April 1. As he has done in past hearings, Holmes sat silently through proceedings. He wore a red jail jumpsuit and sported a thick, bushy beard and unkempt dark brown hair.

His parents, James and Arlene Holmes, sat silently at the front of the courtroom and left without comment.

In the nearly eight months since Holmes first shuffled into court with vacant eyes and reddish-orange hair, neither he nor his lawyers have said much about how he would plead.

Holmes’ lawyers repeatedly raised questions about his mental health, including a recent revelation that he was held in a psychiatric ward for several days last autumn, often in restraints, because he was considered a danger to himself.

That raised the possibility that they could end up entering a plea of not guilty by reason of insanity at the hearing.

Holmes’ lawyers said they were not ready to enter a plea. The plea carries risk, however. Prosecutors would gain access to Holmes’ mental health records, which could help their case if the evidence of insanity is weak.

If Holmes does plead insanity, the proceedings would be prolonged further while he is evaluated by state mental health officials. With the judge entering the plea, prosecutors would not have access to Holmes’ health records.

If a jury agrees he was insane, he would be committed indefinitely to a state mental hospital. There would be a remote and unlikely chance he could be freed one day if doctors find his sanity has been restored.

Prosecutors laid out a case that Holmes methodically planned the shooting for months, amassing an arsenal and elaborately booby-trapping his apartment to kill anyone who tried to enter.

On the night of the attack, they say, he donned a police-style helmet, gas mask and body armour, tossed a gas canister into the seats and then opened fire.

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