TMZ has confirmed a person of interest has been detained by Pima County Sheriff's Department in the abduction of Savannah Guthrie's mom, Nancy.

A law enforcement source involved in the investigation tells us it is not a member of the Guthrie family.

The Pima County Sheriff's Office tells TMZ the individual was detained during a traffic stop south of Tucson.

We broke the story ... our law enforcement sources told us the operating theory is the kidnapper is from the Tucson area and wrote the ransom note that was sent to a couple Tucson TV stations and TMZ.

This morning, the FBI released the first images of Nancy's apparent kidnapper ... and the feds blasted the footage out to the public on the same day they received the info.

As we told you, the abductor is promising to return Nancy to Tucson within 12 hours of receiving the $6 million ransom ... which means they're currently somewhere within a roughly 700-mile radius of Tucson.

There were 2 ransom deadlines, both of which have passed ... even though Savannah tried reaching out to the kidnapper and even offered to pay up.

Tuesday afternoon we noticed activity for the first time in the Bitcoin account listed in the first ransom note that was sent.

Story developing ...