Lawyers for the suspect in last year's US movie theater massacre on Monday changed their client's plea to "not guilty by reason of insanity."

Judge Carlos Samour allowed the alteration from the original standard not-guilty plea for James Holmes, but indicated that he would not decide whether to accept or reject the new plea until after a May 31 hearing.

Defense attorneys for Holmes, who is accused of killing 12 people and injuring 70 others on July 20 at a movie theater in the Colorado town of Aurora, told the court that at least one mental health expert had examined Holmes and made a diagnosis that supports an insanity plea.

The prosecutors, who are seeking the death penalty, objected to the insanity plea for Holmes, who is charged with 166 accounts of murder and attempted murder.

"Our largest frustration is the amount of time it has taken to get to this point," said senior deputy district attorney Jacob Edson, accusing the defense of stalling.

Defense attorney Daniel King argued that the case is complex and voluminous and that his team has been working "on all cylinders" to prepare their plea.

A state mental evaluation might take weeks or months. Should Holmes be found to be sane during the shooting spree, he would face the death penalty. The insanity plea is widely considered as his best chance to avoid capital punishment.

His trial is scheduled for February 2014.