(CNN) -- James Holmes was in "the throes of a psychotic episode when he committed acts that resulted in the tragic loss of life" at a Colorado movie theater a year ago, his attorneys said in a court filing.
It is the first time the defense has admitted that Holmes killed 12 people in the mass shooting, The Denver Post reported.
The judge in Holmes' murder trial last month accepted the defendant's plea of not guilty by reason of insanity.
Holmes previously offered to plead guilty and spend the rest of his life behind bars if authorities would spare his life, but prosecutors in Arapahoe County announced in April they would seek the death penalty.
The defense then decided to make an insanity plea.
Judge Carlos Samour Jr. has told attorneys he expects the trial to take place in 2014.
The burden is on the prosecution to prove that Holmes was sane at the time of the shootings. Attorneys will use the results of a mental evaluation as evidence at the trial.
Holmes faces murder and other charges in the July 20, 2012, shooting spree at the premiere of the Batman movie "The Dark Knight Rises."
CNN's Jim Spellman and Shawn Nottingham contributed to this report.