New Delhi, Feb. 4 -- The search for Nancy Guthrie, the 84-year-old mother of Today show co-host Savannah Guthrie, has entered a critical phase as authorities investigate possible ransom demands, alarming medical-device data and new evidence suggesting an abduction. With the FBI now involved and President Donald Trump offering federal assistance, the case has intensified amid growing fears for the elderly woman's safety.

The case is now a criminal investigation and the search enters its fourth day.

Here are the five key developments in the urgent search for Savannah Guthrie's missing mother, Nancy Guthrie:

The Pima County Sheriff's Department confirmed on Tuesday (February 3) that it is aware of reports suggesting a possible ransom note...