A saboteur deliberately cut the ropes of a young skydiver's parachute, causing him to plunge 13,000ft to his death, police confirmed yesterday.

In an attack that is believed to be unique in British parachuting history, Stephen Hilder, 20, a veteran of more than 200 jumps, died of multiple injuries when both his main and reserve parachutes failed to open.

"The shock is not that somebody could do it, the shock is that somebody has done it. It's pretty horrendous," said Dave Hickling, chief instructor at the British Parachute School, near Nottingham. "You can do all sorts of things to stop parachutes working but whoever did it had the intention of causing a fatality. It was not a joke or a prank - they intended to attack this young man."

Seven other skydivers who leapt from the same SMG Turbo Finist aeroplane last Friday landed safely at Hibaldstow airfield, near Brigg, Lincs - the largest parachute centre in Britain.

But onlookers watched helplessly as Mr Hilder's parachute failed to open when he broke away from the rest of the formation team at the crucial 3,000ft mark and he was sent hurtling to the ground in a cornfield half a mile away.

Detectives said yesterday that both the cord on the main chute and the strapping on the reserve had been cut. They were treating his death as murder, although they would not rule out suicide.

Police are to questioning friends around the country to find out about Mr Hilder's state of mind, but admitted they had no idea why the popular member of the skydiving community appeared to have been targeted.

Det Supt Colin Andrews, from Humberside Police, said: "It is an absolute fact that both parachutes were deliberately tampered with and on the basis of that, we have to strongly suspect that murder was the motive.

He was a popular, intelligent man with a bright future. He has a stable family background which makes the motive for this a mystery.

"One thing we have to consider is that this was a random act and Stephen was in the wrong place at the wrong time.

"It is a tragic waste of a young man with a bright and promising future and it is a particularly horrendous way to die."

University students from across the country travelled to the airfield last week to take part in the annual university parachuting competition.

Mr Hilder, an undergraduate at the Royal Military College of Science at Shrivenham, Wilts, who was planning a career in the Army, attended with three friends but had only been able to make one jump, on Wednesday, because of low cloud and poor weather earlier in the week. His equipment was checked on Wednesday night and found to be in good working order before it was stored in a room at the centre that was left unlocked during the day.

As an experienced skydiver, he would have checked his main parachute himself before the jump on Friday but his reserve chute would also have been looked at by an instructor to ensure the leg and chest straps were fastened properly. However the parachute would not have been completely unpacked.

In a statement, Mr Hilder's family said: "Steve had a tremendous sense of humour and made friends wherever he went. He was a wonderful son and brother, whose place in his very close-knit family will never be filled."

An experienced parachutist, Mr Hilder had jumped in Britain, France and South Africa and was well aware of the risks.

In June he was with a team from Skydive Bristol in the South of France when Elenora Baker, a fellow member, died after colliding with another skydiver above Marseilles.

He began skydiving at Bristol University and it quickly became a passion. When he transferred to the defence academy at Shrivenham last year, he played a major part in revitalising the college's skydiving club.

Among other groups, he belonged to the Army Parachute Association based at Airfield Camp in Netheravon, Wilts, said to be one of the oldest parachute clubs in the world.

Detectives were interviewing anybody who was at Hibaldstow last week and were particularly interested in any photographs, including those taken at a fancy dress party in the centre on Thursday.