Another clue about Nancy Guthrie's potential kidnapper is tucked into an email from a man who claims he knows the individual who abducted Nancy ... and it sounds like the suspect may have crossed the border.

The latest money demand sent to TMZ ends with this ... "for the man hunt of the main individual that can give you all the answers and be prepared to go International."

It sounds to us like the guy demanding the $100,000 FBI reward implies that Nancy's kidnapper is hiding out in Mexico.

Nancy was abducted from her home near Tucson, Arizona ... and Mexico is a short drive south.

It's unlikely the kidnappers would have taken Nancy on a commercial jet ... that would require ID and the jig would be up. Plus, a previous ransom note sent by the alleged kidnapper themselves indicated Nancy was within a 700-mile radius of Tucson, which further supports the theory this kidnapper may be in Mexico.

Our sources tell us the FBI is hyper focused on the ransom note we received ... and they're also very interested in this latest batch of emails demanding bitcoin in return for information on Nancy's kidnapper.

The phrase "the main individual" is also significant ... it clearly suggest there are accomplices.

Harvey Levin broke it all down on CNN in a segment Friday evening ... check out the clip here for the latest twist in this bizarre case.