The man accused of killing 12 people in a crowded US cinema last year, James Holmes, plans to plead not guilty by reason of insanity, according to court documents filed Tuesday by his attorneys.

Holmes is accused over the July 20 massacre at a midnight screening of the Batman movie "The Dark Knight Rises" in the Colorado town of Aurora, outside Denver -- a crime that revived America's long-running debate about gun control.

"Mr Holmes intends to tender a plea of not guilty by reason of insanity," according to the documents, a copy of which was obtained by AFP.

Lawyers for Holmes, who injured another 70 people in the incident, will formally make their request to change his plea at a hearing on May 13 in a court in Centennial, Colorado, according to the document signed by his public defenders.

The trial judge had entered a not guilty plea on Holmes' behalf in March. At that time, his lawyers said they were not yet ready to enter a plea.

In late March, defense lawyers filed an unexpected motion offering that Holmes would plead guilty in return for the prosecution not pressing for the death penalty, but agreeing to a sentence of life in prison without parole.

Prosecutors shot back within 24 hours, accusing the defense of trying to negotiate a plea deal in public, in violation of a gag order on the grisly case. They then said they would seek the death penalty.

Colorado currently has three convicted prisoners on death row but has not executed anyone since 1997.

Witnesses said Holmes threw smoke bomb-type devices before opening fire in the theater with weapons including an AR-15 military-style rifle, a 12-gauge shotgun and a .40-caliber pistol.

When Holmes first appeared in court last year, he had bright orange hair and looked befuddled. He has since let his hair color grow out and now has curly brown hair and a beard.