Prosecutors wanting to prove that a gunman methodically carried out a movie theater shooting in the U.S. that left 12 people dead and wounded 59 others have a difficult task before them: If James Holmes pleads not guilty by reason of insanity, they must prove he is sane.

Unlike other states where the defense needs to prove insanity, prosecutors in Colorado are the ones who have to show that a defendant is sane _ all without the ability of having their own experts examine Holmes.

"It's burden of proof on steroids," said Marcellus McRae, a former federal prosecutor now in private trial attorney in Los Angeles. "It's totally subjective. It's not like proving somebody pulled the trigger. That's objective."

Whether he pleads guilty by reason of insanity, the case against Holmes promises to focus on his mental health.

A court hearing Thursday will examine his relationship with University of Colorado psychiatrist Lynne Fenton, to whom he mailed a package containing a notebook that reportedly contains violent descriptions of an attack.

His attorneys say Holmes is mentally ill and that he sought help from Fenton at the school, where he was a Ph.D. student, until shortly before the July 20 shooting. Prosecutors allege Holmes may have been angry at the failure of a once promising academic career.

Insanity is unlike the mental competency argument used for Jared Loughner, who ultimately pleaded guilty in federal court to the 2011 Arizona shooting that killed six people and wounded 13 others, including then-Rep. Gabrielle Giffords.

Mental competency involves whether a defendant is aware of what's happening in court and can assist his or her attorneys with a defense. In Loughner's case, he received treatment, became competent and entered his plea.

In an insanity defense, a defendant is deemed not guilty because he didn't know right from wrong and is therefore "absolved" of the crime, Jefferson County District Attorney Scott Storey said, who recently lost and insanity case.

If Holmes is found sane and goes to trial and is convicted, his attorneys can try to stave off a possible death penalty by arguing he is mentally ill. Prosecutors have yet to decide whether to seek the death penalty.

Authorities have said that Holmes meticulously stockpiled ammunition and other gear and booby-trapped his apartment to kill, suggesting that he knew what he was doing.

With the burden on prosecutors, those details don't matter, prosecutors say.

Insanity cases depend on a court ordered evaluation by state psychiatrists and up to two additional state evaluations requested by defense attorneys or prosecutors and approved by a judge.

In about 40 states and in federal court, the burden of proof is on the defense, which must prove that a client is insane.

U.S. defense attorneys say the system is fine because a defendant gives up certain rights when pleading insanity, including the right to remain silent. Once a defendant enters his plea, the court orders an independent evaluation by a state doctor, from whom findings of insanity in criminal cases are rare, said Scott Robinson, a Denver criminal attorney.

"It's not an `our doctor, your doctor' thing," Robinson said. "There is a certain built-in advantage for the defense, but only if the opinion from the state evaluation is insanity."

"What they're saying is, `When we don't like what the independent state-hired psychiatrist finds, we want to be able to get our own hired guns and find somebody to say that he is not insane,'" said Denver criminal defense attorney Dan Recht, a former public defender and past president of the Colorado Criminal Defense Bar. "The constitution says, `Sorry, you don't get that in this case or any other case.'"

In federal court, a defendant has to prove by clear and convincing evidence that he is insane.

The uphill battle for prosecutors is compounded by the incomprehensible nature of the crime.

"What do you think the popular mindset is about somebody coming into a theater and killing a bunch of people? That he's crazy," McRae, the former federal prosecutor said. "If they (prosecutors) didn't have to prove insanity this wouldn't be much of a trial at all."