Public defenders for James Holmes, 25, said in the Arapahoe County District Court filing that prosecutors have so far not accepted the offer, which would spare their client the death penalty in one of the deadliest mass shootings in U.S. history.
Holmes faces multiple counts of first-degree murder and attempted murder stemming from the July 20 massacre at a showing of the Batman film "The Dark Knight Rises" in Aurora, Colorado that also wounded 58 people.
Arapahoe County District Attorney George Brauchler has said that he would formally inform the court during a hearing scheduled for Monday whether his office would seek the death penalty against Holmes.
Brauchler has not made his decision public, but in February announced that he had added a death penalty lawyer to the prosecution team.
All three of Colorado's death row inmates were convicted and sentenced in Arapahoe County. At a state legislative hearing earlier in March, Brauchler testified in favor of keeping the death penalty on the books in Colorado.
A spokeswoman for the Arapahoe County District Attorney's Office could not be reached for comment on Wednesday afternoon.
Voice of Russia, Reuters