A strand of DNA recovered from Nancy Guthrie’s Arizona home is undergoing extensive testing at the FBI’s crime lab as the investigation hit the 100-day mark — and authorities continue to hold out hope for a break in the case.

The DNA sample recovered from the 84-year-old’s home in the wake of her abduction had initially been sent to a private lab in Florida by local Pima County sheriff’s detectives, despite the feds offering early on to analyze it at their state-of-the-art facility.

Several weeks after Nancy went missing, the evidence was finally turned over to federal investigators for testing.

Authorities haven’t disclosed exactly what type of DNA is being analyzed, or where it was found, but law enforcement sources told CBS News that the testing was still in the works.

Retired FBI special agent Jason Pack said DNA analysis, which includes building out entire family trees to identify suspects, “takes far longer than television crime dramas would have people believe.”

“That kind of work is slow because it has to be right,” he told the outlet.

“An item containing DNA, such as a strand of hair found somewhere in a house, is one thing, but a strand of hair near the victim’s last known location, such as in her bed, would be a high priority for the FBI,” added Lance Leising, an ex-FBI special agent in Arizona.

Pima County Sheriff Chris Nanos, who has been ridiculed for bungling DNA evidence and shutting the federal agency out of the high-profile probe early on, refused to comment about the sample still being analyzed.

“It would be highly inappropriate of me to speak to the evidence. We have to keep the integrity of this case,” he said.

“If we make an arrest, that individual has the right to a fair trial, I can’t sit here and address all of that.”

“We are working hard with all of our partners to resolve this case, and we will,” he added.

It comes as the search for “Today” show host Savannah Guthrie’s mom hit the 100-day mark on Monday.

Nancy is believed to have been taken from her Tucson home in the early hours of Feb. 1.

Chilling security footage later recovered from her doorbell camera captured a masked man loitering around her doorstep the night police believe she was kidnapped.

Blood spatters were later discovered on her front doorstep.

As the search for the grandmother surpassed its 100th day, investigators still hadn’t determined any credible leads or suspects in the case.