Bryan Kohberger defense: Was his DNA planted at the crime scene?

Defense lawyers for Idaho quadruple murder suspect Bryan Kohberger may be building his alibi around a theory that his DNA was planted on the knife sheath found near one of the victims, to build a "rigged" case against him.

The 28-year-old WSU criminology grad student is accused in the brutal killings of four University of Idaho students—Kaylee Goncalves, Madison Mogen, Xana Kernodle, and Ethan Chapin were found in their off campus apartment house last November.

They had all been stabbed to death and police recovered a Ka-Bar knife sheath near Mogen’s body. Investigators arrested Kohberger at his family’s home in Pennsylvania, after they allegedly discovered Kohberger’s DNA on the sheath.

RELATED: DA says Bryan Kohberger's DNA matches sample from knife sheath near Idaho victim Maddie Mogen

But in a June 23 court filing, Kohberger’s defense team attacked the DNA evidence, and the investigative genetic genealogy (IGG) methods used to match it to Kohberger, as being too convenient to the prosecutor’s case.

"Presumably, the Defense is expected to accept at face value that the sheath had touch DNA just waiting for testing by all the FBI’s myriad resources," wrote Kohberger's attorneys. "Additionally, the Defense is to guess whether the State focused its investigation on Mr. Kohberger via bizarrely complex DNA tree experiment or through its faulty identification of the vehicle involved in this case."

Kohberger’s objection to State’s motion for Protective Order

The defense theory that this was all a setup suggests there may have been a conspiracy among local, state and federal investigators.

"What the State’s argument asks this Court and Mr. Kohberger to assume is that the DNA on the sheath was placed there by Mr. Kohberger, and not someone else during an investigation that spans hundreds of members of law enforcement and apparently at least one lab the State refuses to name," wrote the defense.

They also wondered if there may have been other suspects, saying prosecutors wanted to "prevent Mr. Kohberger from seeing… how many other people the FBI chose to ignore during their investigation."

Prosecutors fired back in a July 14 filing with a scathing response to the idea Kohberger was framed for the murders.

"Defendant theorizes that the lGG process was ‘like lineup where the government was already aware of who they wanted to target.’ Defendant’s ‘rigged lineup’ analogy defies logic," wrote prosecutors.

That’s because the DNA profile was far from the only piece of evidence they used to identify Kohberger as a suspect: "… The IGG information is neither exculpatory nor material to guilt or punishment. The family tree built by the FBl merely pointed law enforcement to Defendant, and law enforcement followed that lead to develop the substantive evidence of guilt that was used for his arrest and that will be used at trial."

Prosecution’s reply in support of motion for Protective Order

The back-and-forth arguments were made leading up to the July 24 deadline for Kohberger to offer an alibi.

Idaho law requires a defendant who pleads not guilty to provide a written timeline of where they were when the alleged crime occurred, and the names of anyone who can support their claim. In court Monday, defense attorneys declined to offer such an alibi statement, though they implied they can prove Kohberger was elsewhere the night of the murders.

"Evidence corroborating Mr. Kohberger being at a location other than the King Road address will be disclosed pursuant to discovery and evidentiary rules as well as statutory requirements," the defense said. "It is anticipated this evidence may be offered by way of cross-examination of witnesses produced by the State as well as calling expert witnesses."

PREVIOUS COVERAGE: Bryan Kohberger might claim 'alibi' in Idaho murders case, court filing reveals

Kohberger’s trial is expected to begin Oct. 2, and he faces the death penalty if convicted.