Accused cinema shooter James Holmes is set to undergo a lengthy mental evaluation after a judge accepted his plea of not guilty by reason of insanity following last summer's (12) Colorado movie theatre massacre.

The suspect, 25, is accused of killing 12 people and injuring 58 after allegedly opening fire in an Aurora movie theatre just before a midnight screening of The Dark Knight Rises in July (12).

Lawyers for Holmes, who has been charged with a total of 166 crimes - including 24 counts of first-degree murder and 116 of attempted murder, had previously offered to enter a guilty plea for their client in exchange for a life imprisonment verdict, but prosecutors rejected the deal because they're planning to pursue the death sentence.

The insanity plea was officially accepted by Judge Carlos Samour Jr. on Tuesday (04Jun13), and Holmes will now have to sit through a series of medical tests, which could take several months, to determine his mental state before the case can proceed to trial.