They say their decision was made because the defense had not provided them with enough information. Prosecutors also said anyone reading news stories about the offer would inevitably conclude "the defendant knows that he is guilty, the defense attorneys know that he is guilty, and that both of them know that he was not criminally insane."
The suspect, James Holmes, is charged with multiple counts of murder and attempted murder in the July 20 shootings in a packed movie theater in the Denver suburb of Aurora. Twelve people were killed and 70 were injured.
Holmes' attorneys disclosed in a court filing Wednesday that their client has offered to plead guilty, but only if he wouldn't be executed. Prosecutors criticized the defense attorneys for publicizing the offer, saying it should have been part of private plea negotiations.
Prosecutors did not say what information the defense refused to give them, but the two sides have argued in court previously about access to information about Holmes' mental health.
The district attorney is expected to announce on Monday whether he will seek the death penalty for Holmes.
The Associated Press contributed to this report.