Nancy Guthrie Case: Forensic Professor Explains Potential DNA Challenges

Savannah Guthrie’s mother Nancy has been missing for more than a month.

There is hope that DNA could help crack the case, but Pima County Sheriff Chris Nanos told NBC News last month that the DNA found in the home was “mixed,” meaning it included genetic material from more than one person.

He explained, “We listen to our lab, and our lab tells us that there’s challenges with it." He said it could take "weeks, months or maybe a year" to get the results.

The “Today” show spoke with Arizona State University Forensic Science Professor April Stonehouse, who is an expert in mixed DNA, to find out more. Stonehouse is not involved in Nancy’s investigation.

What types of challenges might the lab be facing? Stonehouse said, "I think that means that there are potentially items of evidence that maybe don’t have a lot of DNA on them to begin with.”

She added that it could also mean that the mixture is mostly DNA from just one person.

“Let’s say 90 percent of the DNA comes from the victim and only 10 percent is from the suspect,” she said. "What they can potentially do is go back and rerun that sample and add more DNA hoping to bring up the signal from the suspect’s profile.”

People magazine also spoke with CeCe Moore, the chief genetic genealogist at Parabon Nanolabs last week.

Moore explained, “A complex mixture is much more difficult to work with,” adding, "The more people you get in that DNA (sample), particularly unknown people's DNA, the harder it is to just isolate one person's out of that.”

While Moore is not working on the Guthrie case, she has worked with the same lab the Pima County Sheriff’s Department is using.

She told People, “They're very, very good at what they do. So, if they are struggling with it, that tells me it's a very difficult sample.”

Moore is guessing the evidence investigators found is “less likely to be a blood sample or something that is a really good source of DNA.”

A masked person was seen on security camera footage the night Nancy disappeared. 

Moore said, “If I was the kidnapper, I would be extremely worried right now, particularly if I knew there was some kind of altercation, or I knew I touched things in there.”

Based on the security cam footage, Moore said, “It looked like he may have had a bite flashlight in his mouth. When you see him bending over toward the camera, I think it's very possible saliva could have been left because of that."