The National Transportation Safety Board said a re-examination of the craft's last radar traces showed that it plunged towards the sea far more steeply than previously thought.

The craft - piloted by Mr Kennedy - was making a normal descent towards its destination, the island of Martha's Vineyard, off the US east coast, when it began rapidly losing height from 2,200ft.

It fell 1,100ft (330m) in 14 seconds, 10 times the normal rate.

The new information fuels speculation that Mr Kennedy, a relatively inexperienced pilot, had lost control of the plane.

Mr Kennedy, his wife Carolyn and sister-in-law Lauren Bessette are all presumed to have died after the plane disappeared.

Investigators have yet to establish where the aircraft - a single-engine Piper Saratoga II HP - struck the water.

Police divers searching for wreckage ruled out one possible site when they discovered that an object detected by sonar was nothing more than a big rock.

Nine other possible debris sites have yet to be investigated.

George Eykyn reports: "This morning divers have resumed their search"

The navy has also sent in the dive ship Grasp - which was used to search for TWA flight 800 when it crashed in the Atlantic three years ago.

The ship carries a remotely operated submersible and is described as having extensive dive capability.

Investigators are also hoping to recover a cockpit voice recorder fitted to the plane - although it is not known whether this had been activated by the pilot.

Some debris believed to come from the plane has been found washed up on the shore.

Family grief

Grant Stinchfield, with a statement from the Bessette family

On Monday the Kennedy family acknowledged the death of Mr Kennedy, his wife and sister-in-law.

In a written statement issued by Mr Kennedy's uncle, Senator Edward Kennedy, the family said: "His loss leaves an enormous void in all our lives."

Earlier, the Bessette family released their first public statement since the accident.

JFK Jnr and his wife had been travelling to the family home at Hyannisport for the wedding of his cousin Rory when the aircraft disappeared.

Monday's New York Post reported that Mr Kennedy - a recently qualified pilot - had been reluctant to land at Martha's Vineyard to drop off his sister-in-law.

Mr Kennedy said he did not want to have to land twice because of his inexperience as a pilot, his biographer David Heymann said.

US in mourning

The loss of JFK Jnr continues to cause a deep sense of shock across the United States.

During their joint press conference on Monday, President Bill Clinton and Israeli Prime Minister Ehud Barak offered their condolences.

JFK Jnr, 38, a law graduate, was born just two months before his father's inauguration as the 35th US president.

He never entered politics, choosing instead to observe via the pages of his glossy political magazine George.

• What happened to JFK Jnr's plane?
• The Kennedys: A family cursed
• JFK Junior: Born into fame and tragedy
• Picture gallery: Life in the limelight
• Kennedy makes world headlines
• New York's sorrow
• Carolyn: Camelot's Queen