The granddaughter of Robert F Kennedy, brother of former US President John F Kennedy, has died.

US authorities found the body of Maeve Kennedy Townsend McKean on Monday - days after her canoe apparently capsized in a bay between Maryland and Virginia.

Her eight-year-old son Gideon is missing, presumed dead.

McKean (40) was the daughter of former Maryland Lieutenant Governor Kathleen Kennedy Townsend and granddaughter of Robert F Kennedy.

She went missing late Thursday afternoon after the pair went out into waters near the family's home in Maryland.

A preliminary investigation found that McKean and her son may have been paddling the canoe out into the bay to retrieve a ball and were unable to get back to shore.

A search for Gideon is set to resume on Tuesday.

Their eight-year-old son Gideon | Image: Facebook/David McKean

David McKean wrote a long Facebook post on Saturday in tribute to his wife and young son - calling his wife "the brightest light" he had ever known.

He said: "Despite heroic efforts by the Coast Guard and many state and local authorities, the decision has now been made to suspend the active rescue effort.

"The search that began yesterday afternoon went on throughout the night and continued all day today.

"It is now dark again. It has been more than 24 hours, and the chances they have survived are impossibly small.

"It is clear that Maeve and Gideon have passed away."

"The search for their recovery will continue, and I hope that that will be successful."

He added: "I know that people have many questions about what happened as we grapple with this tragedy.

Maeve and Gideon | Image: Facebook/David McKean

"Here is what I have come to understand. We were self-quarantining in an empty house owned by Maeve’s mother Kathleen on the Chesapeake Bay, hoping to give our kids more space than we have at home in DC to run around.

"Gideon and Maeve were playing kickball by the small, shallow cove behind the house, and one of them kicked the ball into the water.

"The cove is protected, with much calmer wind and water than in the greater Chesapeake.

"They got into a canoe, intending simply to retrieve the ball, and somehow got pushed by wind or tide into the open bay.

"About 30 minutes later they were spotted by an onlooker from land, who saw them far out from shore, and called the police.

"After that last sighting, they were not seen again."

On his son he said: "Gideon was 8, but he may as well have been 38.

"He was deeply compassionate, declining to sing children’s songs if they contained a hint of animals or people being treated cruelly.

"He hated if I accidently let a bad word slip.

"He spent hours upstairs reading, learning everything he could about sports, and trying to decipher the mysteries of the stock market.

"But he was also incredibly social, athletic, and courageous."

"Maeve turned 40 in November, and she was my everything. She was my best friend and my soulmate.

"I have already thought many times over today that I need to remember to tell Maeve about something that’s happening.

"I am terrified by the idea that this will fade over time.

"You could hear Maeve’s laugh a block away - and she laughed a lot.

"She was magical - with endless energy that she would put toward inventing games for our children, taking on another project at work or in our community, and spending time with our friends."

The Coast Guard has recovered their canoe, which was capsized.