The Idaho student murders suspect may not have forgotten the Ka-Bar knife sheath found in a bed with two of the four victims – it may have been planted there in an effort to mislead investigators, according to a criminal profiler who has been following the case.

"If you took a pistol out of your holster, wouldn't you put it back in?" said John Kelly, a psychotherapist with experience interviewing serial killers, in an interview with Fox News Digital. "I don't know anybody who wouldn't, and if I went fishing and had to take my knife out, I would put it back in the sheath."

It's a reflexive action, he said, adding that a vegan like murder suspect Bryan Kohberger likely put the knife away somewhere in order to keep the blood from contaminating his clothes or vehicle.

"You're such a clean vegan who's obsessive-compulsive about what you eat and everything else, just the hygiene of carrying a bloody knife around, wearing it somewhere on your person as you get out of the house," he said.

IDAHO MURDERS: BRYAN KOHBERGER'S TRASH MAY BE KEY IN MASSACRE PROBE

Idaho victims last photo

Madison Mogen, top left, rides on the shoulders of her best friend, Kaylee Goncalves, as they pose with Ethan Chapin, Xana Kernodle and two other housemates in Goncalves' final Instagram post, shared the day before the four students were stabbed to death. (@kayleegoncalves / Instagram)

The answer may hinge on what a surviving housemate, the only eyewitness, saw that night.

A police affidavit states that she heard noises, including whimpering and crying, and then saw a masked man with "bushy eyebrows" leaving through a sliding door. It makes no mention of seeing an intruder carrying a weapon.

"This is staging 101."

— John Kelly, criminal profiler

"The girl didn't say anything about seeing a knife," Kelly said. "Did he put it in his clothes somewhere and have blood all over?"

IDAHO MURDERS UPDATE: KAYLEE GONCALVES' FAMILY LAWYER APPEALS GAG ORDER IN BRYAN KOHBERGER CASE

Kelly said the sheath may have been left intentionally – after being wiped clean of fingerprints but not thoroughly enough to remove the touch DNA evidence that police found on the snap.

Bryan Kohberger escorted to court in Pennsylvania

Bryan Kohberger is escorted to court in Pennsylvania prior to his extradition back to Idaho to face charges in the deaths of Madison Mogen, Kaylee Goncalves, Xana Kernodle and Ethan Chapin. Inset: Kohberger's WSU portrait and the last known image of the victims together (The Image Direct for Fox News Digital | WSU | Instagram)

The attacker could have put the knife in a different sheath or some other sealed container and left the "USMC"-stamped sheath behind in an effort to cast suspicion on someone with ties to the military, Kelly said.

"This is staging 101," he continued. "They're going to look at this, and they're going to think it's a military guy that did this – some guy with some kind of training who lives up the road."

INSIDE IDAHO MURDERS SUSPECT BRYAN KOHBERGER'S UNUSUALLY LONG ROUTE HOME TO PENNSYLVANIA

However, police recovered DNA on the snap that they later said matched a familial sample taken from the trash at Kohberger's parents' house 2,500 miles away in Pennsylvania.

University of Idaho victims Madeline Mogen, Ethan Chapin, Xana Kernodle, and Kaylee Goncalves

The victims of the Nov. 13 University of Idaho massacre: Kaylee Goncalves, left, Ethan Chapin, Xana Kernodle and Madison Mogen. (Instagram @xanakernodle / @maddiemogen / @kayleegoncalves)

"He may have thought this was the perfect ruse. Again, he's no genius; his ruse and staging set him up to get caught," Kelly said. 

If he had simply forgotten the knife, police may have found more DNA on the sheath along with fingerprints, Kelly said. As of a gag order on Jan. 3, police had still not recovered the murder weapon.

"That could have precipitated the question: ‘Who else did you arrest?’" Kelly continued, referencing a rumored statement that Kohberger made to police when they took him into custody on Dec. 30. "I think he had to believe that that was going to lead them to somebody, some direction, with the sheath there."

IDAHO MURDERS: BTK SERIAL KILLER ON SOLITARY CONFINEMENT AND BRYAN KOHBERGER, ‘I KNOW HOW HE FEELS’

Kelly's hypothesis is "not unreasonable," said Paul Mauro, a retired NYPD inspector who is also an attorney.

Bryan Kohberger in driver seat

Bryan Kohberger was stopped by Indiana police on Dec 15, 2022, before he was identified as a suspect in the murder of four University of Idaho students. (Hancock County Police Department)

UNIVERSITY OF IDAHO STUDENT STABBINGS TIMELINE

"[The] other side of that would be that he was mentally in an altered state," he told Fox News Digital.

Kohberger, a criminal justice Ph.D. student at Washington State University, is accused of entering a six-bedroom rental home near the University of Idaho campus just seven miles away around 4 a.m. on Nov. 13, 2022.

Inside, police say he ambushed four students, some of whom may have been sleeping, with a large knife.

sliding glass door with lights on at night

A view of the sliding glass door in the rear of the home at 1122 King Road, Moscow, Idaho, on Nov. 14, 2022. (Derek Shook for Fox News Digital)

CLICK HERE TO GET THE FOX NEWS APP

The following day, police found 21-year-old best friends Kaylee Goncalves and Madison Mogen dead in a bed on the third floor. On the second level, they found housemate Xana Kernodle, 20, and her visiting boyfriend, Ethan Chapin, also 20. All four had been stabbed multiple times.

Kohberger faces four counts of first-degree murder and a felony burglary charge. 

He could face the death penalty if convicted. He's being held without bail and is due back in court on June 26.

Judge Megan Marshall issued the initial gag order on Jan. 3, shortly after Kohberger's arrest, restricting comment from prosecutors, the defense, law enforcement and other officials.