Navy divers were trying to discover if the "large target" on the seabed off Martha's Vineyard island could be the missing aircraft of John Kennedy jnr.
He, his wife, Carolyn, and her sister, Ms Lauren Bessette, are now presumed to have died in the crash of the single-engined aircraft last Friday night on its approach to the island airport.
As the search entered its fourth day, a ship using sonar scanning devices, was reported to have located a "large target" at a depth of about 150 ft. Further debris was also picked up, sealed and flown to Otis Air Force Base on Cape Cod where the search operations are being co-ordinated.
Investigators said yesterday that the aircraft went into a steep descent known as a "graveyard spiral" seconds before it crashed. In the last 14 seconds of flight tracked by the radar, it dropped at 4,700 ft a minute. At that speed the plane fell 1,100 ft in 14 seconds, Mr Robert Pearce told reporters.
Meanwhile, it was revealed yesterday that an official search for the aircraft and its three occupants did not begin for four hours after it was first reported overdue at Martha's Vineyard. An airport employee, Mr Adam Budd, was asked by a couple waiting for Ms Lauren Bessette to raise the alarm.
Mr Budd, who is a licensed pilot himself, called a Federal Aviation Administration station on the mainland but he had only sketchy information and no action was taken.
It was only when a member of the Kennedy family made a call to the Coast Guard about four hours later that the alarm was raised and a search got under way.
Mr Kennedy had expressed concern about his ill-fated flight, according to the writer Mr David Heymann, who spoke to him about a week beforehand. Mr Kennedy had complained in light-hearted fashion about having to land first at Martha's Vineyard to drop off Ms Bessette before going on to Hyannisport for a second landing.
"I'm no Charles Lindbergh, I'm not the most experienced pilot in the world," he said, according to Mr Heymann.
A businessman who met Mr Kennedy in Canada several weeks ago has said he was concerned at the flight because his foot was in plaster after a paragliding accident, but the cast had been removed by the time of the flight last week.
The Kennedy family has indicated that it has no plans to have the body of Mr Kennedy buried beside his parents in Arlington National Cemetery outside Washington. This was in response to reports that the much-visited site was being inspected to see if it could hold another grave.
At the Kennedy family compound at Hyannisport, the American flag flew at half mast, as did many other flags in the area. Mr Paul Hill, husband of Ms Court ney Kennedy, helped to lower the flag following the statement from Senator Edward Kennedy on behalf of the family accepting there was no hope the missing three could be found alive.
Members of the family continued to keep a profile at the compound which is made up of three family houses. The tents which had been erected for the wedding of Ms Rory Kennedy have now been dismantled and taken away.
Some of the family have taken advantage of the good weather to take short sailing trips but have avoided contact with the media camped out at the end of the street leading to the compound.
The President, Mrs McAleese, has expressed the sorrow of the Irish people to Ms Caroline Kennedy Schlossberg and Senator Edward Kennedy on the death of Mr Kennedy, his wife Carolyn and her sister, Ms Lauren Bessette.
"I know that John jnr will be profoundly missed, not only by family and friends, but also by those whose lives he touched and who came to know him as a fine individual," the President said. Mrs McAleese added that Irish people would always hold a very special place in their hearts for the Kennedy family.