(CNN) -- Prosecutors said Monday they will seek the death penalty for Colorado movie theater shooting suspect James Holmes despite his offer last week to plead guilty in exchange for avoiding a death sentence.
"It is my determination and my intention that in this case for James Eagan Holmes, justice is death," Arapahoe County District Attorney George Brauchler said.
The decision comes less than a week after defense attorneys filed documents saying Holmes had offered to plead guilty and spend the rest of his life behind bars in exchange for avoiding the death penalty.
Prosecutors took the defense to task for publicly offering it, saying they hadn't been given enough information to even consider such a deal.
"Not only improper, but grossly improper," prosecutors said in a Thursday court filing. "For the intended purpose of generating predictable publicity."
Holmes faces 166 counts of murder and attempted murder for last year's shooting at an Aurora movie theater that left 12 people dead and 58 wounded.
Attorneys on both sides are under a gag order, leaving case watchers to divine tactics from court documents.
"Prior to arraignment, Mr. Holmes made an offer to the prosecution to resolve this case by pleading guilty and spending the rest of his life in prison, without any opportunity for parole," the documents filed by the defense team read.
Last month, a judge entered a standard plea of not guilty for Holmes, who is accused in the July 20 shooting.
In the documents filed Wednesday, his attorneys said they were still exploring a mental health defense, "and counsel will vigorously present and argue any and all appropriate defenses at a trial or sentencing proceeding, as necessary."
Federal agents have said the former University of Colorado doctoral student planned the attack for months.
Authorities say Holmes booby-trapped his apartment with explosives, then traveled to the movie theater armed with the weapons, tear gas and body armor planning to kill audience members during a screening of "The Dark Knight Rises."
Witnesses who spoke to CNN said the gunman roamed the theater, shooting randomly as people tried to scramble away or cowered between seats.
CNN's Jim Spellman, Ed Payne and Dana Ford contributed to this report.