FBI Agent's Stark Warning: Nancy Guthrie Case 'Much Harder' Without 'Meaningful' Leads

A former FBI agent says it's getting harder to get the Nancy Guthrie investigation going as the search approaches its seventh week.

The search for Nancy Guthrie is not making headway, not as hoped for by her family and the authorities. Despite the latest updates, the mounting discoveries have yet to break the case. One expert says it gets even harder now that Guthrie has been missing for 50 days.

Investigation Needs New, Meaningful Leads

Lance Leising, a retired FBI supervisory special agent, said it all boils down to 'a lack of meaningful leads.' 'That's the initial thing it says to me,' he told CBS News. 'It becomes much harder to keep the investigation going, keep it current and fight for new leads.'

At present, the search's success hinges on two key leads, the trace DNA and footage of a potential suspect stalking the residence. Leising said he remains hopeful about the case's prospects, but asserted that 'it takes a while' to process the evidence.

'Maybe you've got multiple people within the same DNA family that could potentially be suspects. You have to investigate each one of those independently,' he said, referring to the mixed DNA found at Guthrie's home.

'A complex mixture is much more difficult to work with,' CeCe Moore, Parabon Nanolabs Chief Genetic Genealogist, told People. '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.'

Forensics Struggle with DNA Evidence

Pima County Sheriff Chris Nanos told NBC News the process could take 'weeks, months or maybe a year.' 'We listen to our lab, and our lab tells us that there's challenges with it,' he confirmed.

The FBI has recovered more camera images of the Guthrie residence. They have also retrieved hours of Tucson-area surveillance footage. The ski-masked person caught by Guthrie's doorbell camera could mean foul play, especially after a source claimed the same person was recorded in two separate incidents.

Doorbell Footage Suggests More DNA Traces

Moore said the footage could be damning evidence against the person of interest. 'It looked like he may have had a bite flashlight in his mouth,' she explained. 'When you see him bending over toward the camera, I think it's very possible saliva could have been left because of that.'

'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,' she added. 'I think it would be extremely difficult to be in a location for approximately 40 minutes and not leave your DNA behind, even with the way he was dressed.'

Authorities have sifted through more than 1,500 potential leads via their tip line, boosted by the £751,000 ($1 million) reward offered by the Guthrie family. The investigation has yet to make its first breakthrough, and it's been more than a month since the last law enforcement news conference.

'I'd like to be hopeful that — and I'm sure the family is incredibly hopeful — that the silence is because they're on to something,' Leising stated. 'They just need to investigate harder.'