Prosecutors in the US will seek the death penalty against James Holmes in the murder of 12 people during a showing of the Batman film "The Dark Knight Rises" last year.
Mr Holmes, 25, is accused of opening fire inside a Denver cinema during a midnight screening of the film last July
He is charged with multiple counts of murder and attempted murder in the shooting rampage, which also wounded 58 others.
Another dozen people suffered non-gunshot injuries as they fled the Aurora, Colorado, cinema.
Arapahoe County District Attorney George Brauchler had previously announced he had assigned a death penalty lawyer to the prosecution team.
In court documents released on 28 March he rejected a defence offer to let Mr Holmes plead guilty and serve a life sentence if capital punishment were taken off the table.
Arapahoe County District Judge William Sylvester entered a not guilty plea for Mr Holmes last month but said he would consider allowing that to be changed to not guilty by reason of insanity.
Last week, public defenders said in a court filing that Mr Holmes was willing to plead guilty and serve a life sentence without the possibility of parole if prosecutors agreed not to seek to have their client executed.
While Mr Holmes' attorneys said they are prepared to mount an insanity defence, they wrote in the filing that "Mr Holmes is currently willing to resolve the case to bring the proceedings to a speedy and definite conclusion for all involved."
Mr Brauchler, in a written response, called the move by the defence improper at this stage of the case and "that it was filed for the intended purpose of generating the predictable pre-trial publicity."
"The only conclusion that an objective reader would reach ... is that the defendant knows that he is guilty, the defence attorneys know he is guilty and that both of them know that he was not criminally insane," Mr Brauchler wrote.
In court pleadings, public defenders Daniel King and Tamara Brady have said Mr Holmes has been hospitalised twice since his arrest, once for "potential self-inflicted injuries."
At one point, prison officers determined that Mr Holmes was a danger to himself and in "immediate need of a psychiatric evaluation."
He was transported by ambulance to a Denver psychiatric ward "where he was held for several days, frequently in restraints," his lawyers wrote.