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 its state-of-the-art facility. Several weeks after Nancy went missing, the evidence was finally turned over to federal investigators for testing. A strand of DNA recovered from Nancy Guthrie’s Arizona home is still undergoing extensive testing at the FBI’s crime lab as the investigation hit the 100 day mark. Getty Images 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.…