David McKean knows the chances are “impossibly small” that his wife and 8-year-old son — both members of the Kennedy family — are still alive after disappearing in the Chesapeake Bay two days ago.

Even as authorities continued to search for Maeve Kennedy Townsend McKean, 40, and son Gideon Joseph Kennedy McKean, David McKean acknowledged it was a recovery mission in a moving tribute to them on Facebook on Saturday.

“It is clear that Maeve and Gideon have passed away,” wrote McKean, in a post confirmed as authentic by a close friend of the family. “The search for their recovery will continue, and I hope that will be successful.”

Maeve and Gideon have been missing since Thursday afternoon, when they climbed into a canoe to retrieve a lost ball from a waterfront home south of Annapolis.

The family had temporarily moved out of their house on Capitol Hill in Washington, McKean said, and into the Shady Side, Md., home of Maeve’s mother, former Maryland lieutenant governor Kathleen Kennedy Townsend. There was more room for them there while isolating for the coronavirus outbreak.

McKean said his wife and son, who attended Watkins Elementary School in the District, were playing kickball behind the house near a shallow cove, described as protected from the Greater Chesapeake “with much calmer wind and water.”

One of them kicked the ball into the water, and Maeve and Gideon climbed into a canoe “intending simply to retrieve the ball, and somehow got pushed by wind or tide into the open bay,” McKean said.

A close friend who spoke on behalf of the family, Alan H.H. Fleischmann, Maeve’s godfather, said neither was wearing a life jacket, anticipating they would be in the canoe only a minute or two. Maeve apparently underestimated the wind, gusting about 30 mph, and the strong undertow that sucked the canoe into the bay and into two- to three-foot-high waves, Fleischmann said.

Someone called police around 4:30 p.m. on Thursday to say he saw the canoe from the Columbia Beach pier. The Coast Guard said mother and son were “seen struggling to return to shore in a canoe” near Herring Bay and then not seen again.

McKean said the canoe was found Friday evening, miles away and capsized.

Authorities from the Maryland Natural Resources Police, the U.S. Coast Guard and the Anne Arundel County Fire Department began their search Thursday using boats and helicopter crews. By Friday night, searchers had covered 2,275 square miles of water.

The Natural Resources Police said on Saturday that they were back out on the Chesapeake Bay using side-scan sonar and were being assisted by helicopters from Anne Arundel County and state police. A spokeswoman for the Natural Resources Police confirmed the mission is now deemed a “recovery effort.”

The apparent deaths have sent another shock wave through a family likened to Great Britain’s royalty and one that has been beset by tragedy.

Maeve McKean was a granddaughter of the late senator Robert F. Kennedy and grandniece of former president John F. Kennedy.

Her mother, Kathleen Kennedy Townsend, was a teenager when her father, Bobby Kennedy, was assassinated, and she has now lost a daughter and grandson. The McKeans have two other children, 7-year-old Gabriella and 2-year-old Toby.

In a statement, Townsend described Gideon as a star athlete “who loved soccer, golf, and running,” and who “took after his parents in the most extraordinary ways. He loved riddles, math, chess, and adventures.”

Though he was 8 years old, his father said, he “may as well have been 38.” In his Facebook post, McKean described his son as “deeply compassionate, declining to sing children’s songs if they contained a hint of animals or people being treated cruelly.”

Gideon, he said, “hated if I accidentally let a bad word slip,” and spent hours in his room reading “and trying to decipher the mysteries of the stock market.”

In his school picture, Gideon insisted as many friends as possible join him in the shot. He played sports six days a week and complained when his father hadn’t signed him up to play on the seventh. Gideon was named after a Supreme Court case ordering states to pay for defense attorneys for the poor.

“It is impossible to sum up Gideon here,” wrote his father, a human rights attorney. “I am heartbroken to even have to try. I used to marvel at him as a toddler and worry that he was too perfect to exist in this world. It seems to me now that he was.”

Maeve McKean was the executive director of the Georgetown University Global Health Initiative and was president of the Parent Teacher Association at the Capitol Hill Cluster School. Her mother said she had a “fire emanating from her soul.”

She graduated from Boston College and Georgetown University with a joint degree in law and international conflict resolution and served in the Peace Corps in Mozambique. She met her husband while working for Sen. Dianne Feinstein (D-Calif.). During the Obama administration, Mc­Kean worked in the State Department’s global AIDS program and on human rights in the Department of Health and Human Services.

John T. Monahan, a senior adviser to the president of Georgetown University, said in a statement that Maeve “was a brilliant, passionate, and energetic advocate for human rights and social justice.”

Maeve turned 40 in November, and McKean said “she was my best friend and my soul mate.” He said her laugh could be heard blocks away, “and she laughed a lot.”

His wife simultaneously took on community projects, raised the children and trained to run in the Boston Marathon. She once returned from a 30-hour trip to Asia and took a cab from the airport to a pool to play with the children, he said.

“There were weeks when we had people over to our house so often that our kids would be confused when we were just having dinner as a family,” McKean said.

He said his two other children, Gabriella and Toby, slept next to him Friday night.

“I promised them that I would do my best to be the parent that Maeve was,” he said, “and to be the person that Gideon clearly would have grown up to be.”

Dan Morse contributed to this report.