Air accident investigators were today trying to find out why a plane nosedived into a Sussex field, killing all four people on board.

Police said it could take up to a week to formally identify the victims, one of whom is believed to have lived in the Billingshurst area.

The Rockwell Commander plane was flying from Shoreham Airport to Biggin Hill, Kent, when it crashed near the village of Sharpthorne, just south of East Grinstead.

Witnesses said a wing snapped as it fell just 18 minutes into Saturday afternoon's flight, hitting the ground 100 yards from a secluded farmhouse.

The light aircraft smashed into pieces on impact, killing its pilot, co-pilot and two passengers.

Police sealed off the scene as investigators sifted through the wreckage before it was loaded on to a lorry and taken to specialist facilities at Farnborough in Hampshire for detailed examination.

The pilot is thought to have been a 55-year-old man from Kingston in Surrey. His co-pilot was 40 and lived in France. Police believe the other passenger was from Watford, Herts.

The bodies were taken to the Princess Royal Hospital, Haywards Heath, where post mortems will take place.

Inspector Mike Pollitt, of Sussex Police, said: "Identification, by very nature of the accident, is going to be quite difficult and therefore it may take some time."

Nicolette Ashby and her husband, Russell, who own Newcoombe Farm, were returning home when they received a call from the farm-hand alerting them to the tragedy near their home.

Mrs Ashby said: "It looked like tissue paper all over the field. There was a wing and that was all I could recognise as a plane. It looked as though it had nosedived and the front was buried in the ground. It appeared to have broken up into smithereens."

Neighbour Stephen Garside said: "It seemed to hit a snow cloud. There was a noise and it just nosedived.

"There was no explosion. The starboard wing was bent completely back as it came down."

Janet Rivers, who lives close to the scene, said: "I heard a strange noise. You could tell something was in trouble."

The Rockwell took off from Shoreham at 2.07pm and the alarm was raised at 2.25pm.

A Shoreham Airport spokeswoman said: "The plane was not registered here, it was literally just passing through. We have no information about the people on board."

In December, Peter Bevan, 43, and his daughter Lauren, 11, from Billingshurst, were killed when their Piper Arrow light plane crashed in Berkshire.