State prosecutors hope a judge will order Christian C. Nielsen to undergo a psychological examination.

Nielsen, who is charged with four counts of homicide, might defend himself through his lawyer by arguing he was insane at the time of the slayings, said William Stokes, chief of the Maine Attorney General’s criminal division.

Wanting to cover all bases, Stokes said Wednesday he is seeking an evaluation of Nielsen’s mental health soon, as close to the time of the killings as possible. A hearing on the state’s motion is set for Oct. 18 in Oxford County Superior Court.

Nielsen, 31, is accused of killing Julie Bullard, 65, the owner of the Black Bear Bed and Breakfast in Newry; Bullard’s daughter, Selby Bullard, 30; her friend, Cynthia Beatson, 43; and a guest, James Whitehurst, 50, over Labor Day weekend.

According to police reports, Nielsen burned and buried Whitehurst’s body and shot and dismembered the three women.

In Maine, a defendant can claim insanity as a defense. The law says someone is “not criminally responsibility if, at the time of the criminal conduct, as a result of mental disease of defect, the defendant lacked substantial capacity to appreciate the wrongfulness of the criminal conduct.” In other words, mental illness kept the defendant from understanding his criminal actions were wrong.

The law goes on to define “mental disease or defect” as “severely abnormal mental conditions that grossly and demonstrably impair a person’s perception or understanding of reality.” That might include hearing voices telling the defendant to commit the crime because the victim is evil, Stokes said.

Under the law, the use of drugs and alcohol by themselves, at the time of the crime, don’t constitute a “mental disease or defect.”

Stokes said that only the court can compel the defendant to undergo an exam. On the other hand, Nielsen’s lawyer might already have had his client evaluated, Stokes said. That would put prosecutors at a disadvantage if they didn’t have one of their own, he said. Nielsen’s attorney, Ron Hoffman, could not be reached for comment Wednesday.

Maine law also allows a defendant to divide his trial into two parts: in the first, the state would have to prove that the defendant committed the act; in the second, the defendant would have to show he wasn’t criminally responsible.

When claiming an insanity defense, the defendant must prove “by a preponderance of evidence,” that he was not criminally responsible, Stokes said.

The insanity defense is “relatively common” in Maine, Stokes said, used in roughly 20 percent of the cases his office prosecutes.

Authorities allege that Nielsen has admitted to the killings. An insanity defense is more likely in cases in which the identity of the assailant is not contested, Stokes said.

He said the circumstances surrounding this case, where four people are dead, raised in his mind the possibility of an insanity defense.

Daniel Lilley, a Portland defense lawyer, said he has used the insanity defense twice at roughly 40 murder trials, most recently about 25 years ago.

He recommends it only in rare cases, he said, because “even those on the edge would prefer to take their chances with a jury.”

Although juries generally think a defendant might go free a week or a month after successfully using an insanity defense, Lilley said his experience tells him otherwise.

“Trying to get someone out of an institution in this state is well-nigh impossible,” Lilley said. One of the two clients he represented using an insanity defense was successful and is still confined to an institution some 30 years later, Lilley said. The other was convicted.