In the weeks after four University of Idaho students were found slaughtered in a house near campus last November, a growing roster of investigators desperately searching for answers had yet to identify a suspect or even find the murder weapon.
Publicly, the authorities were assuring worried residents in the small college town that they were making progress. Privately, they were exhausting their prospects, scouring through the backgrounds of those with the thinnest possible connections to the case.
Through the first two weeks of December, investigators put some of their focus on classmates of the victims; they also widened the search to examine a man in another state who had been known to send harassing messages to women but had visited Idaho only twice in his life. They looked at a woman previously charged with assaults in the region. They looked at a man once accused of wielding a knife. They looked at sex offenders. They looked at a white supremacist. Each turned out to be a dead end.
Then, after spending weeks sifting through an array of evidence that seemed to lead nowhere, investigators announced an arrest in late December on the other side of the country: Bryan Kohberger, a Ph.D. student from a nearby university. He was identified only after investigators turned to an advanced method of DNA analysis that had rarely been used in active murder investigations.
The story of how dozens of officers from local, state and federal agencies took the quadruple murder investigation into extraordinary territory is only now becoming more apparent, through recently obtained records and interviews with people familiar with the investigation who discussed key details that emerged before the issuance of a gag order in the case.
The case has shown the degree to which law enforcement investigators have come to rely on the digital footprints that ordinary Americans leave in nearly every facet of their lives. Online shopping, car sales, carrying a cellphone, drives along city streets and amateur genealogy all played roles in an investigation that was solved, in the end, as much through technology as traditional sleuthing.
Now indicted on four counts of murder, Mr. Kohberger has declined to enter a plea but contended previously through his lawyer that he would be exonerated. Investigators have yet to detail a possible motive: To this day, family members of the victims remain unaware of any prior connections that the accused killer had with the four young people who were killed.
The process of identifying and arresting a suspect took over six weeks — weeks of increasing frustration and painstaking examination of evidence as the community pushed for answers.
The first 911 call came in around noon, some seven hours after the murders. Madison Mogen, 21; Kaylee Goncalves, 21; Xana Kernodle, 20; and Ethan Chapin, 20, had been stabbed to death overnight in their bedrooms. Initially, the Moscow Police Department described the attack as “targeted” and assured residents that there was no risk to the public. But with no indication of who had committed the attack or why, the authorities eventually backtracked.
“We cannot say that there is no threat to the community,” the police chief, James Fry, said at a news conference on Nov. 16, three days after the killings.
The department, which had only a few dozen officers, enlisted the Idaho State Police and the F.B.I. to help, bringing in dozens of additional investigators. Forensic teams processed evidence from the bedrooms, snapped photos around the home’s back door and hunted for footprints; others searched for surveillance video from around town.
A week after the killings, records show, investigators were on the lookout for a certain type of vehicle: Nissan Sentras from the model years 2019 to 2023. Quietly, they ran down details on thousands of such vehicles, including the owners’ addresses, license plate numbers and the color of each sedan.
But further scrutiny of the video footage produced more clarity, and on Nov. 25 the police in Moscow asked law enforcement agencies to look for a different type of car with a similar shape: white Hyundai Elantras from the model years 2011 to 2013.
Just across the state border, at Washington State University, campus police officers began looking through their records for Elantras registered there. Among those they found was one registered to Mr. Kohberger, who had moved to the area earlier in 2022 to pursue a Ph.D. in criminology.
They checked Mr. Kohberger’s car more closely, according to a court affidavit, including visiting the parking lot near his apartment in the early morning hours of Nov. 29. But the vehicle was a 2015 model, not the earlier models being sought in Moscow.
The next day, records show, investigators in Moscow were also looking closely at another vehicle: Ms. Goncalves’s car. She had purchased a 2015 Range Rover in the days before her death. Detectives gathered information on the registration, history and legal owners, clearly interested in the history of who had owned the car. But once again, the information provided few answers.
With students at the University of Idaho preparing for final exams, there was a permanent sense of apprehension in the air, as people began locking their doors, acquiring pepper spray and asking on social media how such a brazen killer could remain undetected for so long in a town that had not seen a murder in seven years.
The hunt broadened as investigators vacuumed up more records and data. They had already sought cellphone data for all phones that pinged cell towers within a half-mile of the victims’ house from 3 a.m. to 5 a.m., according to search warrant filings. They collected victims’ bank records, email correspondence and social media account data.
On Dec. 5, detectives made a new request for surveillance video, seeking all footage recorded on U.P.S. delivery trucks in the days before and after the killings.
The next day, after getting back data on Ms. Goncalves’s account on the Tinder dating app, detectives asked for details on 19 specific account-holders, including their locations, credit card information and any “private images, pictures or videos” associated with the accounts.
The request allowed the inquiry to go much broader than an ordinary search for information, using a single warrant to target 19 different people, said Orin Kerr, a law professor at the University of California, Berkeley.
“You couldn’t get one warrant to search 19 houses,” he said.
Investigators were also working with a key piece of evidence: a Ka-Bar knife sheath, branded with a U.S. Marine Corps logo, that had been found next to two of the victims. They initially began looking for local stores that may have sold the weapon, and then fanned out.
A request to Amazon sought the order histories of account holders who had purchased such knives. A follow-up request to eBay focused on a series of specific users, seeking their purchase histories. Some had connections to the area — including one in Idaho and two in Washington State — while others were from far away, including an account in Japan. Because of redactions, it is unclear if Mr. Kohberger’s name came up in those records.
The overall data track was growing enormous, demonstrating the extensive breadth of data on consumer platforms that can be made accessible to law enforcement agencies when they are at the stage of hunting for bread crumbs.
“You could say this is uncharted territory,” Mr. Kerr said. “They are novel legal questions when there are so many records out there.”
At the same time, DNA from the scene was being processed. Forensic teams had examined the knife sheath and found DNA that did not belong to any of the inhabitants of the house. They ran the sample through the F.B.I.’s database, which contains millions of DNA profiles of past criminal offenders, but according to three people briefed on the case, they did not get a match.
At that point, investigators decided to try genetic genealogy, a method that until now has been used primarily to solve cold cases, not active murder investigations. Among the growing number of genealogy websites that help people trace their ancestors and relatives via their own DNA, some allow users to select an option that permits law enforcement to compare crime scene DNA samples against the websites’ data.
A distant cousin who has opted into the system can help investigators building a family tree from crime scene DNA to triangulate and identify a potential perpetrator of a crime. In one of the best-known uses of genealogy in solving a cold case, detectives in 2018 were able to arrest a suspect in a string of rapes and murders years before in California that had been attributed to the so-called Golden State Killer.
Once a suspect is identified, a direct genetic comparison can be used as confirmation before making an arrest.
With few formal regulations, the use of mass genealogy databases has raised concerns about privacy and whether limits should be placed on how the method is used. But authorities across the country say they have found potential in the system to produce leads that are unattainable through traditional investigative efforts.
Barbara Rae-Venter, a genealogy consultant who worked on the Golden State Killer case, said there was now growing interest in using genealogical DNA not just in cold cases but also in active crime investigations.
“That is why the Idaho case is so interesting,” she said.
To do the genealogical analysis, the Idaho State Police contracted with a private company, Othram, in Texas, which had a lab able to produce a more extensive DNA profile from the knife sheath than the state lab was set up to examine.
F.B.I. personnel worked with the profile that Othram had produced, according to two people familiar with the investigation, spending days building out a family tree that began with a distant relative.
By the morning of Dec. 19, records show, investigators had a name: Bryan Kohberger. He had a white Elantra. He was a student at a university eight miles from the murder scene.
Mr. Kohberger was already out of town, his semester complete. He had driven home with his father to Pennsylvania, stopped twice by the police along the way for what officers said was tailgating. Just as agents began to scour his background more thoroughly, he had been terminated from his position as a teaching assistant, after what was described as altercations with a professor.
On Dec. 23, investigators sought and received Mr. Kohberger’s cellphone records. The results added more to their suspicions: His phone was moving around in the early morning hours of Nov. 13, but was disconnected from cell networks — perhaps turned off — in the two hours around when the killings occurred.
Four days later, agents in Pennsylvania managed to retrieve some trash from Mr. Kohberger’s family residence, sending the material to the Idaho State Police forensic lab. Checking it against their original DNA profile, the lab was able to reach a game-changing conclusion: The DNA in the trash belonged to a close relative of whoever had left DNA on the knife sheath.
Mr. Kohberger was arrested on Dec. 30.
“We have an individual in custody who committed these horrible crimes, and I do believe our community is safe,” the police chief, Mr. Fry, announced. “But we still do need to be vigilant, right?”