How did investigators use DNA profiling to identify the suspect in the Idaho student killings?

Police investigating the murders of four University of Idaho students have said they used DNA to link evidence found at the crime scene to their suspect, 28-year-old Bryan Kohberger.

Kohberger was arrested Friday in Pennsylvania, more than six weeks after the Nov. 13 murders of Ethan Chapin, 20, Madison Mogen, 21, Kaylee Goncalves, 21, and Xana Kernodle, 20, in a Moscow, Idaho rental house. He appeared in court Thursday and was read the murder and burglary charges against him.

His court-appointed public defender in Pennsylvania, Jason A. LaBar, told the Today show that Kohberger has maintained his innocence: "He said this is not him, he said he believes he is going to be exonerated."

The wait for more information on the case and the identification of a suspect stirred panic in the small college town. Newly released court filings detail how authorities zeroed in on Kohberger using DNA even as he returned to his family's home state in December.

Here's how they did it.

Investigators used DNA to identify Kohberger

According to an affidavit made public on Thursday, authorities were able to locate DNA of a single male source that was left on the button of a knife sheath found near one of the victims.

Investigators also used surveillance video to link a vehicle they believed to be the suspect's and cell phone location to Kohberger.

They got a warrant to go through trash from Kohberger's family home in Pennsylvania, and obtained DNA from the trash, which they were able to identify with a high level of probability as belonging to the biological father of the person whose DNA was on the knife sheath.

The methods and technology used to arrest Kohberger were conventional DNA profiling techniques using what's known as Short Tandem Repeat (STR) analysis, used for everything from identifying criminal suspects to conducting paternity tests, according to Daniele Podini, chair of the Department of Forensic Sciences at The George Washington University.

What kinds of DNA can be used to profile a potential killer?

DNA is present in every single cell in the body, and it can be deposited at a crime scene in a number of different ways, including through saliva, blood and sweat.

In this case, Podini said the DNA found on the knife sheath could have come from a number of different types of cells or a combination, including what's known as touch or trace DNA that involves a small amount of skin cells.

"When we touch an object, we deposit some of our cells. They may be some skin cells, there may be saliva cells because we just coughed on our hand," Podini said.

How did they link DNA to the suspect's father?

Every person inherits 50% of their DNA from their biological mother and 50% from their biological father.

Podini said investigators likely used items from the family home's trash that came into contact with a single person, like a plastic spoon or a razor blade, to extract and analyze the DNA on them.

"They evidently found a profile that was consistent with being the biological father of the DNA profile obtained from the knife (sheath), which means that at least for every region that they analyzed, half of that was present in both individuals," Podini said.

How the science has evolved, allowing more killers to be caught

In the nearly four decades since DNA profiling was introduced, advancements in science and technology have led to more accurate analysis, Podini said.

Different segments of DNA are now able to be analyzed. Polymerase chain reaction (PCR) can now make copies of small amounts of DNA to make it detectable and analyzable. And, the tools to analyze DNA have become more sensitive, so samples from touch DNA with less information on it, for example, can be used.

Today, researchers are focusing on improving the type of information that can be gleaned from a genetic profile, with the aim of determining someone's eye color or skin pigmentation, for example, Podini said. Current advancements also hope to refine a technique called DNA methylation to determine the age of a person from whom a sample was collected. Scientists also want to come up with methods to more accurately analyze DNA mixtures, when multiple people's DNA are found on surfaces.

The rise of genealogy sites

Technological advancements in DNA forensics—and the rise of genealogy sites such as 23andMe and Ancestry.com—have led to the solving of decades-old cases, including the arrest and capture of the "Golden State Killer" in California, who operated in the 70s and 80s and was arrested in 2018.

In that case, police had a DNA sample from an old crime scene but no suspect to match it to. Decades later, they conducted a familial search using a public database of records from private testing companies uploaded by people who hope to find matches with potential relatives.

Then, they constructed a family tree of the potential relatives of people the original DNA sample matched, and narrowed down by the suspected age of the suspected killer and known location at the time of the killings. That led them to Joseph James DeAngelo, who ultimately pleaded guilty.

After a suspect is identified through that process, more traditional analysis is done using STRs between the sample from the crime scene and a new sample from the individual suspect, usually obtained through a warrant. Police wiped DeAngelo's vehicle door handle to get his DNA while he was inside a store.

The way investigators found the Golden State Killer and the suspect in the Idaho slayings was quite different, Podini said, because of the different DNA analysis methods used involving different technology and sets of genetic markers. Though Kohberger, too, will likely have his DNA from a cheek swab compared to what was found on the knife sheath now that he is in custody to confirm a perfect match, Podini said.