The FBI has taken over an investigation into Nancy Guthrie, the missing mother of US TV host Savannah Guthrie, after a deadline for her ransom expired.
The 84-year-old has been missing since January 31, with her family releasing a series of increasingly desperate pleas for her safe return.
And late on Monday night, local police confirmed the probe into her whereabouts had been passed over to the Bureau.
In a statement, the Pima County Sheriff's Office in Arizona confirmed agents were now "handling all aspects related to any ransom note(s) or communications involving the Guthrie family".
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That came after it admitted it had neither identified any suspects nor people of interest in its investigation.
But in a video address on Monday, Today show host Savannah released an emotional video begging for her mother to be handed back.
"We believe that somehow, someway she is feeling these prayers and that God is lifting her even in this moment," the breakfast TV host said.
"We believe our mum is still out there."
Savannah Guthrie asked her millions of followers to do anything they could to help the search in a tearful plea
|INSTAGRAM/SAVANNAHGUTHRIE
Savannah then asked her millions of followers to do anything they could to help the search by reporting anything they thought may be suspicious.
"We need your help. Law enforcement is working tirelessly around the clock to bring her home, trying to find her. She was taken and we don't know where. And we need your help.
"If you see anything, if you hear anything, if there is anything at all that seems strange to you, that you report to law enforcement.
"We are at an hour of desperation. We need your help."
The ransom note had demanded the Guthrie family pay $6million (£4.4million), and gave a deadline of Monday at 5pm MT (12am GMT) - which prompted Savannah and her siblings to say over the weekend they would be willing to pay for their mother's return.
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PICTURED: Nancy and Savannah Guthrie on the Today show set | GETTY
The Guthrie family had earlier said they will pay for the return of their 'valuable' mother
| INSTAGRAM/SAVANNAHGUTHRIEHowever, authorities have not yet confirmed the note is real.
Blood found on Nancy's front porch was identified as a match to hers - while her doorbell camera was disconnected in the early hours of Sunday morning.
Meanwhile, FBI agents and local police were seen at the Guthrie residence on Friday - at least the second time they searched the home after finding the blood.
Drone footage from Fox News captured one officer poking a long pole into a septic tank near the home, while reports said a vehicle, believed to be Nancy's, was towed away.
Authorities have been seen at the Guthrie residence since Nancy's disappearance on January 31
|REUTERS
Both the Guthrie family and US authorities have voiced concerns over her health because she needs prescription medication every day.
She is believed to have a pacemaker, and has experienced high blood pressure and heart issues.
Savannah Guthrie, who has fronted NBC’s Today programme for 14 years, has not been on-air since her mother's disappearance.
She was also forced to cancel her travel to Italy to cover last week's opening ceremony of the Milan-Cortina Winter Olympics.