James Holmes changes plea to not guilty through insanity over Batman cinema massacre in Aurora, Colorado that killed 12

The cinema shooter - at a premiere screening on the Dark Knight Rises - killed 12 and injured 70

James Holmes has pleaded not guilty through insanity over the Batman cinema massacre in Aurora, Colorado, last year (AP)

The man accused of killing 12 and injuring 70 when he opened fire on a crowded cinema has pleaded not guilty through insanity.

James Holmes, a 25-year-old former neuroscience graduate student, is accused of spending months gathering weapons and ammunition, scouting a cinema and booby-trapping his apartment.

On July 20 last year, an armour-clad gunman slipped into a midnight screening of the newly-released The Dark Knight Rises at the Century cinema in Aurora, Colorado. He then tossed two hissing gas canisters and walked up the aisle firing the gun.

No motive has emerged in nearly 10 months of hearings, but Holmes' lawyers have repeatedly said their client is mentally ill. He was being treated by a psychiatrist before the attack.

A judge entered a standard not guilty plea on Holmes' behalf in March, and he needs court permission to change it. Holmes' lawyers have held off on changing his plea until now, fearing a wrinkle in the law could cripple their ability to raise his mental health as a mitigating factor during the sentencing phase.

Holmes' lawyers announced last week that Holmes would ask to change his plea at the hearing. It is seen as his best chance of avoiding the death penalty.

The insanity plea carries risks for both sides. Holmes will have to submit to a mental evaluation by state-employed doctors, and prosecutors could use the findings against him.

"It's literally a life-and-death situation with the government seeking to execute him and the government, the same government, evaluating him with regard to whether he was sane or insane at the time he was in that movie theatre," said lawyer Dan Recht, a past president of the Colorado Criminal Defence Bar.

Prosecutors must convince jurors beyond a reasonable doubt that Holmes was sane. If they fail, state law requires the jury to find him not guilty by reason of insanity.

If acquitted, Holmes would be committed to the state mental hospital indefinitely.

The mental evaluation could take weeks or months. Evaluators will interview Holmes, his friends and family, and if Holmes permits it, they'll also speak to mental health professionals who treated him in the past.