(NewsNation) — Idaho stabbing suspect Bryan Kohberger has not yet entered a plea but prosecutors are already saying he will not be able to use the not guilty by reason of insanity plea.

During an appearance on NewsNation’s “Prime,” Former CIA and FBI agent Tracy Walder discussed how the insanity defense has not been available in Idaho since 1982, when the state abolished its use.

“He actually cannot use it in Idaho,” Walder said. “I think we’re so used to hearing about it, because many states do allow it.”

That doesn’t mean his mental state can’t be taken into account in court, Walder said.

“And in this case, I think his mental state may mean something to the defense,” she sadi. “However, they can’t use it in a plea. And we have to be really careful with that. Because just because you may suffer from a mental illness of some sort doesn’t make one a murderer.”

Kohberger will have a preliminary hearing in late June, when prosecutors will try to show a judge that they have enough evidence to justify the felony charges. The 28-year-old Washington State University graduate student is charged with four counts of first-degree murder and burglary.

The Nov. 13 slayings of Madison Mogen, Kaylee Goncalves, Xana Kernodle and Ethan Chapin left the rural community in Moscow, Idaho, grief-stricken and afraid, prompting nearly half of the university’s students to leave town for the perceived safety of online courses.

Weeks went by without a named suspect and few details were released, but on Dec. 30 Kohberger, a graduate student studying criminology from the university located just 10 miles (16 kilometers) away — was arrested at his parents’ home in eastern Pennsylvania. Kohberger was extradited to Idaho last week.

University of Idaho students returned from winter break to start classes last week, many of them coming together for the first time since news of the attack spread across campus.

A general feeling of relief was in the air, university spokesperson Jodi Walker said on Wednesday.

“The students are back and enrollments are looking good,” Walker said. “I think everybody’s happy to be back under the circumstances. They’re relieved that an arrest has been made, and ready to focus on the semester.”

The Associated Press contributed to this report.