AQUINNAH, Mass. (AP) _ Divers set out in boats today and a high-tech vessel with side-scan sonar scoured the bottom of the ocean to find out what happened to the plane that carried John F. Kennedy Jr., his wife and her sister.

The Kennedy family, meanwhile, stayed in seclusion and kept a flag flying at full staff.

Divers planned to search at least two locations pinpointed by the Rude, a National Oceanic and Atmospheric Administration research vessel.

Ten state police divers in gray shirts and blue shorts boarded two boats about noon at Menemsha Harbor in Chilmark, on the west end of the Vineyard. A third vessel left carrying oxygen tanks and dive suits.

``We're on our way out,'' said state trooper Blake Gilmore, a dive team leader.

The search for Kennedy's missing plane, which has covered almost 9,000 square miles, on Sunday produced no major finds but authorities were focusing on ``a couple of targets'' identified by the Rude's sonar, Coast Guard Rear Adm. Richard M. Larrabee.

``These are simply potential targets and don't necessarily represent an aircraft or part of it,'' he said Sunday. He said the targets were in water 60 to 80 feet deep. The Coast Guard wouldn't comment this morning on whether the Rude (pronounced Rudy) had found anything else overnight.

Late Sunday, the operation's definition was changed from ``search and rescue'' to ``search and recovery'' _ a minor revision on the surface, but full of meaning for those who held out hope that the three were still alive.

After two days of searching, investigators made the announcement that the three were presumed dead in waters off Martha's Vineyard.

``I have spent some very painful moments with the families tonight,'' said Larrabee said. ``I think they understand. They have been very appreciative of what we have been trying to do.''

The plane piloted by Kennedy, 38, and carrying his wife, Carolyn Bessette Kennedy, 33, and her sister Lauren Bessette, 34, was reported missing early Saturday morning.

Kennedy took off from New Jersey on Friday night on a trip to his cousin Rory's wedding Saturday on Cape Cod. The plane was to make a stop on Martha's Vineyard to drop off Lauren Bessette before continuing to the Cape.

The wedding was postponed after the plane was reported missing. Instead, the family held a Mass to pray for the missing.

This morning the family held another Mass at the compound, where the American flag continued to fly at full staff.

Larrabee noted that it would be difficult for a person to survive more than 18 hours in the area's 68-degree waters.

``We know that the aircraft was not equipped with any survival equipment,'' he said.

Investigators had few clues. The search has turned up bits of debris, including a piece of Lauren Bessette's luggage, but no substantial wreckage. A radio signal that searchers had hoped was an emergency beacon from the plane turned out to be a false alarm.

As the nation agonized over the fate of the glamorous son of America's political royal family, hundreds of people used airplanes, boats and all-terrain vehicles to search for any evidence of the plane or its occupants.

The Kennedys stayed close to their Hyannis Port compound. Some walked on the beach Sunday afternoon. Ethel Kennedy, the widow of Sen. Robert F. Kennedy, joined other family members who went boating.

Kennedy's sister, Caroline Kennedy, 41, the last survivor of Camelot, waited out the search with her husband, Edwin Schlossberg, and their three children at their home on New York's Long Island.

She had been extremely close to her brother. As children together in the brief years of John F. Kennedy's administration, they were about 6 and 3 years old when their father, the nation's 35th president, was assassinated in 1963.

The disappearance of his son, publisher of George magazine, sparked the same type of emotional, around-the-clock news coverage that accompanied the death two years ago of Princess Diana.

Mourners left flowers at the Brookline, Mass., birthplace of his father, at the John F. Kennedy Library and Museum in Boston and on the steps of John and Carolyn Kennedy's apartment building in New York City.

Some experienced pilots said that a relatively new pilot like Kennedy _ who received his pilot's license last year _ may have been ill-advised to fly at night, while others said it was routine.

A source friendly with the Kennedy family told The Associated Press that Kennedy had intended to fly earlier in the day, but his sister-in-law, an investment banker, had to work late.

Jim Hall, chairman of the National Transportation Safety Board, said Sunday the crash investigation could take six to nine months.

Linda Tilden, who was vacationing on Martha's Vineyard, remembers watching images of John F. Kennedy Jr. as a little boy in a short coat, saluting his father's casket in 1963.

Now she must explain to her two school-age daughters what happened to Kennedy, just as her parents explained the assassination of his father to her.

``I told them there was an accident, and that it is a sad time,'' said Tilden, 41. ``That adults will be sad.'