The NBC Today show presenter's 84-year-old mother disappeared from her Arizona home on February 1, and police have been treating her disappearance as a kidnapping.
Guthrie has previously admitted she believes Nancy is no longer alive but she is adamant she will not give up the search until she is able to bring her mother home.
In a post on Instagram to mark Mother's Day in the US, Guthrie wrote: "We will never stop looking for you. We will never be at peace until we find you.
"We need help. Someone knows something that can make the difference. Call 1800CALLFBI. You can be anonymous, and the reward remains available. Please keep praying. Bring her home."
Guthrie previously confessed she had accepted Nancy was no longer alive after receiving a message from a higher power.
During an interview with her Today show colleague Hoda Kotb, Guthrie cried as she explained: "Early on, I felt that I heard, one of the very few times in my life, I did hear God speak to me.
"As I said to myself: 'I can handle anything. I just can't handle not knowing ... I have to know'. And I heard a voice and it said: 'You do know where she is. She's with me.'
"So whether she's on this earth still or whether she's in heaven, I know where she is. I know who she's with. But we need to know. "
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