Kathleen Kennedy Townsend's daughter and eight-year-old grandson disappeared after boating on the South River in Maryland, Governor Larry Hogan told reporters.
Two members of the Kennedy US political dynasty, including a grand-niece of assassinated president John F Kennedy, were missing on Friday after they went canoeing.
Kathleen Kennedy Townsend's daughter and eight-year-old grandson disappeared Thursday after boating on the South River in Maryland, Governor Larry Hogan told reporters.
An intensive search has been underway since Thursday.
"On behalf of the people of Maryland, I expressed our most heartfelt sympathies and prayers to her and to her entire family during this difficult time," Hogan said.
Kennedy Townsend, 68, is a former Maryland lieutenant governor, and the daughter of Robert F Kennedy, the onetime US attorney general in his brother's administration.
The iconic American clan has had more than its share of tragedy.
RFK was himself slain in 1968 as he campaigned for president five years after his brother's death.
In 1999 John F Kennedy Jr and his wife and sister-in-law died when the small plane he was piloting crashed off the Massachusetts coast.
Saoirse Kennedy Hill, a granddaughter of Robert F Kennedy, died of what was ruled an accidental drug overdose last year at age 22.