"Prior to arraignment, Mr. Holmes made an offer to the prosecution to resolve this case by pleading guilty and spending the rest of his life in prison, without any opportunity for parole," the documents filed by the defense team read.
Last month, a judge entered a standard plea of not guilty for Holmes, who is accused in the July 20 shooting.
In the documents filed Wednesday, his attorneys said they are still exploring a mental health defense, "and counsel will vigorously present and argue any and all appropriate defenses at a trial or sentencing proceeding, as necessary."
The case against Holmes
Federal agents have said the former University of Colorado doctoral student planned the attack for months.
He began buying guns last May, allegedly building an arsenal of two Glock handguns, an AR-15 rifle, a shotgun and 6,295 rounds of ammunition.
Authorities say Holmes booby-trapped his apartment with explosives, then traveled to the movie theater armed with the weapons, tear gas and body armor planning to kill audience members during a screening of "The Dark Knight Rises."
Witnesses who spoke to CNN said the gunman roamed the theater, shooting randomly as people tried to scramble away or cowered between seats.
"This is not a 'whodunit.' Everybody knows that James Holmes committed these horrible murders. The question is what punishment he will get," said CNN senior legal analyst Jeffrey Toobin.
"If they (defense attorneys) can get life in prison, they will consider this a victory."
Holmes' trial date has been set for Feb. 3.