NBC's Today show co-host Savannah Guthrie says her family is living an unending nightmare nearly six months after her mother's disappearance, pleading with her kidnappers to "tell us where to look for her".A video released on Monday, local time, is the latest public plea from the journalist since Nancy Guthrie, 84, disappeared from her home just outside Tucson, Arizona on February 1. She said her family would never stop looking for her."The world can be a cruel and unforgiving place, and I know that you have tried to do things the right way," she said."So I'm asking you, begging you, to do the right thing now. Make the right choice. Help us find her. Tell us where to look for her. I truly believe it is never too late to do the right thing."Authorities believe Nancy Guthrie was taken against her will on February 1. They found blood near the front doorstep of her home just outside Tucson, and the FBI later released surveillance videos showing a masked man on the porch that night.Volunteers and search teams scoured nearby desert terrain filled with cactuses, bushes and boulders in the weeks after she vanished. A volunteer group recently searched for her body near the Arizona-Mexico border. Three notes gained wide attentionAuthorities have not said how many purported ransom demands were made after Guthrie's disappearance, though three news organisations reported receiving such notes and turned them over to investigators.The FBI has discounted some of the ransom notes as nothing more than extortion attempts, but said it was still evaluating others that might be legitimate. The federal agency hasn't specified which ransom notes might be genuine.The US television personality has previously made a series of social media posts about her mother after she went missing. In the days after the disappearance, the broadcaster and her siblings acknowledged they were aware of media reports of a supposed ransom letter and were ready to talk and listen, but wanted proof their mother was alive. Weeks later, Savannah Guthrie said her family was offering a $US1 million ($1.45 million) reward for information leading to the recovery of her mother. She also acknowledged for the first time that her mother might not be alive, but added that there was still hope.AP