India, June 3 -- More than four months after Nancy Guthrie disappeared from her Arizona home, Pima County Sheriff Chris Nanos is defending the pace of the investigation, saying complex forensic testing and legal procedures are contributing to delays in the high-profile case.

Speaking to Tucson-based KOLD-TV, Nanos said the investigation relies heavily on laboratory work and scientific protocols rather than quick arrests.

"It's just not a detective goes out there, talks to somebody, and we can make an arrest," Nanos said. "This is a very sensitive case, but what really makes it prolonged is we do rely on labs."

Guthrie, 84, is the mother of NBC Today co-host Savannah Guthrie. Authorities have not publicly identified a suspect, announced...