The urgent search for Nancy Guthrie, the mother of “Today” show host Savannah Guthrie, has entered its fifth day, as her children continue to plead for her safe return.

Nancy Guthrie is believed to have been abducted in her sleep from her Arizona home early Sunday, authorities said. No suspect or person of interest has been identified in the case, and authorities do not know where she is or whether she was targeted, Pima County Sheriff Chris Nanos has said.

The Pima County Sheriff’s Department is set to hold a briefing on the case at 1 p.m. ET on Thursday.

Nancy Guthrie, 84, was last seen at her home in the Catalina Foothills area, north of Tucson, on Saturday night, according to the Pima County Sheriff’s Department. Her family reported her missing on Sunday around noon local time after she failed to show up to church, authorities said.

Savannah Guthrie and her siblings made an emotional plea for their mother’s return in a video message posted to social media on Wednesday.

“Everyone is looking for you, Mommy, everywhere,” Savannah Guthrie said in the video message. “We will not rest. Your children will not rest until we are together again.”

Nancy Guthrie is described as having some physical ailments and limited mobility, but does not have cognitive issues, according to the sheriff. She takes medication that if she doesn’t have in 24 hours, “it could be fatal,” Nanos said.

“Our mom is our heart and our home,” Savannah Guthrie said in the video. “She is 84 years old, her health, her heart is fragile. She lives in constant pain. She is without any medicine. She needs it to survive. She needs it not to suffer.”

The FBI is helping in the investigation. The agency is sending additional agents and experts to Pima County to help reinforce efforts on the ground and to aid local investigators, sources told ABC News on Wednesday.

The sheriff’s department said it is reviewing possible ransom notes as part of the investigation. ABC Tucson affiliate KGUN said it received one of the letters, which it forwarded to law enforcement. Officials say they are investigating if any of these letters are legitimate.

Addressing reports of a ransom letter, Savannah Guthrie said Wednesday, “As a family, we are doing everything that we can. We are ready to talk. However, we live in a world where voices and images are easily manipulated. We need to know without a doubt that she is alive and that you have her. We want to hear from you and we are ready to listen. Please reach out to us.”

Anyone with information is urged to call 911 or the Pima County Sheriff’s Department at 520-351-4900.