Savannah Guthrie's plea Monday to her mother's kidnappers contained a very intriguing phrase -- "We have done our part" -- and now we know what she meant.

Our FBI sources tell us, they knew in advance Savannah was going to post another plea to the kidnappers. Our sources say it was an issue of timing -- this week marks 6 months since Nancy's kidnapping, and, as one FBI source tells us, "Anniversaries are the time to go back to the public and reignite interest [in a case]."

As for what Savannah meant by "We have done our part," we have checked the Bitcoin address from the first ransom note and no money has been recently deposited (The FBI deposited several hundred dollars months ago in an attempt to trace the account, to no avail). Our FBI sources say they continue to work with the Guthrie family and they have not sent the kidnappers money.

Our FBI sources say Savannah was referencing the $1 Million reward for the recovery of Nancy. Obviously, the kidnappers would not expect to get a reward, but we're told she was speaking more broadly to anyone who knows what happened to Nancy.

One FBI source added this stark reality ... "We've made no progress here. We have no idea where Nancy is or who kidnapped her."