(CNN) -- The judge in James Holmes' murder trial on Tuesday accepted his plea of not guilty by reason of insanity.
The suspect in last summer's shooting spree that killed 12 people and wounded dozens at an Aurora, Colorado, movie theater will be taken to the Colorado Mental Health Institute at Pueblo for evaluation.
He's expected to be returned to the Arapahoe County Jail on August 2.
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 the mental evaluation as evidence at the trial.
Samour's ruling Tuesday came about a month after Holmes' attorneys asked to change his plea from a standard not guilty. The original not-guilty plea was entered by a judge on Holmes' behalf in March, over his objection, his defense team said.
The defendant 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.
Asked by the judge Tuesday whether he had any questions about the plea, Holmes replied in a clear voice, "No."
Samour also ruled Tuesday that a package Holmes mailed to his psychiatrist before the shooting can be processed as evidence.
He sent Dr. Lynne Fenton $400 in burned $20 bills, a sticky note with an infinity sign, and a spiral notebook containing a placard. Written on it: "James Holmes, Of Life," according to court documents.
The defense had previously argued that the contents of the package is covered by doctor-patient confidentiality. But with an insanity plea, that confidentiality no longer applies.
Before the shooting, Fenton, who works at the University of Colorado's medical campus, warned campus police that Holmes was dangerous.
Holmes faces murder and other charges in the July 20 shooting spree at the premiere of the Batman movie "The Dark Knight Rises."