We want to make requests to colleagues in the United States and Israel and other countries who have experience working at such depth to help us investigate whether it is possible to lift the black box

Vladimir Rushailo, head of investigation

And many bits of wreckage which could prove whether missile, terrorist attack or malfunction brought the plane down may also be lost forever at the bottom of the sea.

Within a day of the crash, the head of the Russian investigation had asked for international help to help raise the recorders, admitting the operation posed serious difficulties.

Many of the early clues have come from the pieces of wreckage found floating at the scene, some punctured with holes - which may be the only direct evidence the investigators ever get their hands on.

Russian recovery vessels have been racing against time to pull as many pieces from the water as possible before strong currents sweep them away.

Jigsaw-puzzle

Wreckage is strewn over an area with a radius of 10km (6.2 miles) but each fragment could eventually prove vital in the jigsaw-puzzle which investigators must try to assemble.

Terrorist experts believe the 1988 Lockerbie bomb was timed to go off when the plane was over the Atlantic because the killers knew it would make the inquiry virtually impossible.

Relatives will want clear answers on the cause

As it was, flight delays meant the bomb detonated over land, giving investigators the chance to gather every fragment of the plane.

Their painstaking work in reconstructing the plane and its contents allowed them to identify fragments of clothing from the suitcase containing the bomb, which in turn led them to the killers.

Scattered

This time the investigators are not so fortunate.

The pieces of wreckage will be scattered across the bottom of the Black Sea, in waters at least 10 times as deep as the Barents Sea recovery site of the Kursk submarine, which sank in August 2000.

Flight recorders can withstand impact and immersion, and emit beacon signals to guide their recovery

Chris Yates, aviation security expert

Even for an operation at that depth - 100m - the Russians were forced to call in outside help.

In the Black Sea, the problems will be multiplied. The recovery will be seeking not a giant intact submarine, but multiple fragments of wreckage.

Unlike at the Kursk scene, divers cannot operate because the water is too deep even for rigid suits.

Highly corrosive

And fewer than 10 manned submersible vessels in the world are capable of operating at such depths, Chris Funnell of Jane's Underwater Technology told BBC News Online.

Most have been built for scientific research, and would be able to pick up only small pieces of wreckage.

Seabed operations

TWA Flight 800 - crashed into Atlantic off New York, 1996 - fuel tank explosion blamed EgyptAir Flight 990 downed in Atlantic off Massachusetts, 1999 - pilot blamed Air India Flight 182 blew up off Ireland, 1985 - bomb

Two are owned by Russia - the Mir 1 and 2. Ukraine - being blamed by the US for accidentally shooting down the plane - is believed to have another, and France and Japan also have them.

But even recovered wreckage could yield few clues, said David Learmount of Flight International magazine, as the waters of the Black Sea are believed to be highly corrosive, and could make tests on recovered fragments much harder.

Recovery agenda

First priority in the investigation will be given to tracing the flight recorders.

"Some people are suggesting the recorders would not have survived - I would disagree," said Chris Yates, Aviation Security Editor for Jane's Information Group.

"These devices can withstand impact and immersion, and emit beacon signals to guide their recovery."

If the boxes are recovered, the cockpit voice recorder could provide the most vital clues, experts believe.

The explosion witnessed by a passing pilot may have been picked up on the cockpit microphones, allowing experts to analyse the type of sound, and possibly where it came from.

The flight data recorder would also be scrutinised for clues.

Unthinkable

Reports that holes have been found in a recovered door could prove vital - investigators will be able to determine which way the metal had bent and therefore whether a gun had been fired from inside the plane.

A gun or bomb on board would mean the unthinkable had happened: that Israel's watertight airport security had been breached.

When you are talking about these depths, this is not a trivial type of job

Cliff Funnell, Janes Underwater Technology

For further clues, investigators will have to turn their attention to reaching the seabed.

Their first step, believes Cliff Funnell, is most likely to be using a sonar device over the crash area.

It would be lowered from a survey ship and towed back and forth across the area until it found significant debris parts.

The next step could be to lower an unmanned vessel - or Remotely Operated Vehicle (ROV), which would supply pictures of the scene to the ship via fibre-optic cable.

Only then might the hugely complex task of sending a manned submersible to the scene be considered.

Titanic help?

A vessel would first have to be brought to the Black Sea, probably on a purpose-built ship, and a complex operation would follow.

"We are talking about deep ocean salvage," said Mr Funnell.

"When you are talking about these depths, this is not a trivial type of job."

Specialist knowledge from the oil and gas industries could prove key to operating at such depths, he said.

Even operations like the Titanic exploration could provide vital experience.

The mystery could take months or years to solved - and the full picture may never be known.