The BBC's Nikki Albon looks back at the day's events

Hours after the search began, the US Coast Guard confirmed that a piece of luggage and parts of a plane had been discovered on a beach on the Massachusetts coast.

The debris included:

• a wheel
• a headrest
• a support strut

A bag was found with a business card attached bearing the name Lauren Bessette - Mr Kennedy's sister-in-law, who was also onboard the aircraft.

Beachgoers who found the bag said it contained a hairdryer and women's clothes.

The single-engine Piper Saratoga II HP was being flown by Mr Kennedy, who gained his pilot's licence last year.

The plane was also carrying his wife, Carolyn Bessette Kennedy. They were heading for a family wedding in Martha's Vineyard on Friday night when the plane disappeared in hazy conditions.

The BBC's Mark Devenport says footage from radar screens shows the plane was making a normal descent towards the island when it plunged from 2,200ft to 1,300ft in just 12 seconds. Then it disappeared from radar screens.

Manufacturer's statement

The Florida-based company that manufactured the aircraft, The New Piper Aircraft, Inc, said a representative had been dispatched "to assist authorities should an investigation be initiated".

The BBC's Jane Hughes: "News of the disappearance has sent a collective shudder around America."

"The New Piper Aircraft, Inc, is saddened by this situation and is standing by to work diligently with authorities," a company statement said.

Coastguards said they picked up a signal from an emergency beacon in the sea off the eastern tip of Long Island at 0215 local time, along Mr Kennedy's presumed flight path.

The beacon emits an audio signal which can mean the plane has crashed. It can also be set off manually as a distress signal.

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Rescue crews were dispatched immediately, but no subsequent signal was heard.

A spokeswoman for plane's manufacturers said there could have been "an initial signal on impact", but added that the beacon "won't transmit a signal underwater".

Kennedy wedding

Mr Kennedy was going to the wedding of a cousin Rory - the daughter of the late Senator Robert F Kennedy, in Hyannisport.

The wedding, due to take place on Saturday before 275 guests, was postponed as the search began.

Family members gathered at the home of Rory's mother, Ethel Kennedy, with about 50 people and three priests.

They prayed "for the safety of their loved ones'' said family spokesman Brian O'Connor.

President Bill Clinton, who is spending the weekend at Camp David in Maryland, was told about the disappearance of the Kennedy plane by his chief of staff early on Saturday.

Press Secretary Joe Lockhart said the president was being kept informed of the progress of the search.

Massive search

Mr Kennedy's plane took off from Caldwell, New Jersey, at about 2045 (0145GMT) and was due to arrive at Martha's Vineyard a couple of hours later.

The BBC's Richard Quest: "The currents in the water are light and the weather is excellent."

Pentagon officials said 16 aircraft from five states were involved in the search operation, together with four rescue helicopters and two coast guard boats.

A coast guard said the plane was last detected on radar just miles west of Martha's Vineyard.

The weather was very hot and hazy, but flying experts said conditions were not unusually difficult.

The Federal Aviation Administration said no flight plan had been filed as far as it knew.

The Kennedy family contacted the FAA when the plane, a Piper Saratoga, was overdue.

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

But unlike other family members, Mr Kennedy never entered politics.

In 1995 he launched the glossy US political magazine, George - subtitled "not just politics as usual".