In December, investigators traced Mr. Kohberger to his parents’ house in Pennsylvania. On Dec. 27, while he was there over winter break, the police managed to retrieve some garbage from the house and sent what appears to have been a DNA sample from Mr. Kohberger’s father for testing. The results showed a strong probability that the elder Mr. Kohberger was the father of whoever left DNA on the knife sheath.
In a predawn raid on Dec. 30, the police broke through windows and doors of the family home and arrested the younger Mr. Kohberger. They followed up quickly with searches of his apartment in Pullman, the white Hyundai Elantra he had driven with his father to Pennsylvania and the parents’ home. They also got a court order to obtain a direct DNA sample from Mr. Kohberger.
Friends of the victims have been searching for possible connections between the victims and the accused killer — so far, disclosing none — and classmates of Mr. Kohberger’s at Washington State University have examined their own recollections to try to identify clues.
Some said Mr. Kohberger spent time studying the exact kind of techniques that the police used in recent weeks to identify him, and had a deep interest in criminal psychology and crime scenes.
Benjamin T. Roberts, a fellow graduate student at Washington State, said Mr. Kohberger had been interested in areas like psychology and Rational Choice Theory, which suggests that offenders may often try to assess the potential costs and benefits of committing a crime.
“He took the field of study very seriously,” Mr. Roberts said.
But peers also said he at times caused conflict in the program. Mr. Roberts recalled that Mr. Kohberger tended to be more forceful and condescending in challenging the ideas of female students during discussions in classroom settings.
“There was a consistent pattern in which he would push back more with women colleagues than with male colleagues,” he said.
One new revelation in the court affidavit had a tinge of irony: After enrolling in the Ph.D. program at Washington State in August, Mr. Kohberger had applied for an internship. In an essay as part of the application, he described his interest in helping rural police departments collect and analyze data as part of public safety operations. The internship he applied for was at the Pullman Police Department, whose officers would wind up helping in the investigation of the murders.
Kirsten Noyes and Susan C. Beachy contributed research.