After six weeks, 20,000 tips and a 2,500-mile cross-country journey, authorities finally arrested a suspect in the quadruple murder of University of Idaho students — thanks in part to DNA evidence.

Bryan Kohberger, 28, was nabbed Friday in a pre-dawn raid at his parents home in Albrightsville, Pa. after he and his father drove cross country from Washington State for Christmas.

But as they were making the journey, police were already on their tail, according to reports. 

“Sometime right before Christmas we were zeroing in on him being in or going to Pennsylvania,” an unnamed law enforcement source told CNN.

Authorities started trailing Kohberger during the drive east, CNN reported, and an FBI surveillance team tracked him for four days as law enforcement and prosecutors worked to obtain an arrest warrant, two sources told the network. 

Police apparently took DNA from the scene, ran it through a public database, and then used genetic genealogy techniques to connect the DNA to Kohberger through his family members, CNN said.

“What most likely happened is that the crime scene was a mess and there was DNA evidence left all over the place,” Pete Yachmetz, a security consultant and former FBI agent, told The Post Sunday. “So what they did was retrieved all the DNA evidence they could and analyzed it.”

That process, Yachmetz explained, takes time, as authorities first identify the DNA of the victims and then “start looking for DNA of someone who should not have been there.”

In investigations like the Idaho murder, the DNA samples are often put through CODIS, or the Combined DNA Index System, the national DNA database maintained by the FBI, Yachmetz explained.

“What I think took a long time was the massive amount of evidence they collected,” he said. “You just don’t put a droplet under a microscope and then you can analyze it.”

Cellphone data revealed that Kohberger, 28, was often in the same location as students Kaylee Goncalves, Madison Mogen, Xana Kernodle and Ethan Chapin, who were murdered on Nov. 13, a source close to one of the case’s investigators told The Daily Mail.

The alleged killer had been pursuing a doctorate in criminal justice at Washington State University in Pullman, Washington, less than 10 miles from Moscow, Idaho, where students lived. The murder weapon has yet to be located, police said Friday.

Few details have been released on what exactly led police to the PhD student, as the probable cause affidavit remains sealed until the suspect appears in Idaho court, per state law.

The arrest warrant listed four counts of first-degree, with additional charges likely to be filed. Kohberger’s defense lawyer says his client is innocent and will voluntarily allow police to extradite him from Pennsylvania to Idaho.

Moscow Police Captain Anthony Dahlinger said Sunday that more than 400 people called in with tips following his department’s Friday press conference announcing the arrest. 

Yachmetz suspects that Kohberger’s white Hyundai Elantra will be the next major focus for police, who will treat the car “like a crime scene.”

The father and son apparently got the car serviced on Dec. 16, CNN reported, but that shouldn’t prevent the FBI from being able to find trace evidence — like hairs or fibers— and blood stains, which can be uncovered with a luminol test, he said. 

There is no indication Kohberger’s father was involved in the murders.