CENTENNIAL, Colorado (CNN) -- The sanity of James Holmes, the Aurora, Colorado, movie theater shooting suspect, may be a major issue at his arraignment Tuesday.
In court documents, Holmes' attorneys have suggested that they may enter a plea of not guilty by reason of insanity for the shooting rampage at the theater that left 12 people dead and 58 injured on July 20.
Prosecutors have not said whether they'll pursue the death penalty against Holmes. But an insanity plea could make such a move harder, said David Beller, an attorney who is not connected to the case.
"There are a few reasons they wouldn't go for the death penalty; the most important one being his mental state," Beller said. "The Supreme Court, and really society, has been very clear: We don't execute people who are mentally ill."
Family members of some of those who died in the shooting are not happy.
Jessica Watts, whose cousin was killed, said she does not believe Holmes is insane.
"Absolutely not. This was months and months of planning and thousands of dollars spent on his part in order to pull this horrific night off," she said.
Federal agents have said Holmes began buying guns in May 2012, two months before the attack. He allegedly built an arsenal of two Glock handguns, an AR-15 rifle, a shotgun and 6,295 rounds of ammunition.
In addition, authorities contend, the former University of Colorado doctoral student dyed his hair fiery orange and apparently visited the AMC movie theater, taking photographs of hallways and doors, two weeks before the shooting.
According to the Colorado Bar Association, an insanity defense refers to "a person who is so diseased or defective in mind at the time of the commission of the act as to be incapable of distinguishing right from wrong with respect to that act is not accountable."
If Holmes' enters such a plea, he would waive all medical confidentiality and will have to turn over the name of any doctor or psychologist who may have treated him, according to Colorado law.
"If he enters the not guilty by reason of insanity plea, he's going to be examined by state doctors and any statement he makes to those state doctors are given to the prosecution for potential use later," Beller said.
On Monday, a judge ruled that Holmes will also have to agree to be drugged by doctors to assess his condition if he enters an insanity plea.
Earlier this month, Holmes' lawyers tried to have Colorado's insanity defense laws changed.
The attorneys asked the judge to rule parts of the state's insanity defense laws unconstitutional.
Among other issues, they cited the requirement that a defendant "cooperate" with examining psychiatrists as a violation of the defendant's privilege against compelled self-incrimination.
Holmes is charged with a total of 166 counts of murder, attempted murder and other charges.
Authorities say he booby-trapped his apartment with explosives, then traveled to the movie theater armed with four weapons, tear gas and body armor planning to kill audience members during a screening of "Batman: The Dark Knight Rises."
Witnesses who have spoken to CNN about the shooting have said the gunman roamed the theater, shooting randomly as people tried to scramble away or cowered between seats.
Among the 41 calls to 911, one stands out. In the 27-second call, at least 30 shots can be heard amid the chaos.
At his preliminary hearing in January, police who responded described hellish scenes inside the theater and described finding Holmes, dressed in body armor, standing outside, seeming "detached from it all," according to Officer Jason Oviatt.
At the conclusion of the brief hearing, the father of one of the victim's shouted out, "Rot in hell, Holmes."
-- Jim Spellman reported from Colorado; Lateef Mungin wrote from Atlanta.
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