What caused the accident?

The extent of the damage discovered by rescuers has led experts to conclude that the submarine was hit by a large explosion or that it collided with the sea bed or another large vessel. Some theories suggest a collision triggered an explosion.

Two explosions were heard at the time of the accident, by US and Norwegian authorities monitoring Russian exercises.

The second explosion was reported to be equivalent to two tonnes of TNT and bigger than the first.

The theories about the cause of these explosions abound, and so far none have been substantiated:

• A torpedo in the Kursk's forward compartment - which was carrying up to 30 warheads - exploded.

  "There may have been an explosion in one of the weapons systems aboard, for example a torpedo, which then triggered a bigger explosion two minutes later," said Norwegian armed forces spokesman Brigadier Kjell Grandhagen.

  Russia's official military newspaper Krasnaya Zvezda has reported that the Kursk's battery and propeller torpedo-launching technology had been replaced with a cheaper and potentially dangerous liquid fuel system, against the wishes of Navy officials. The liquid fuel is highly combustible.

  US naval experts also believe volatile fuel could be to blame. Using data obtained from an intelligence gathering ship and two US nuclear submarines that were monitoring the Kursk during exercises, they say the first explosion involved fuel from a torpedo or a long-range anti-shipping missile carried by the sub.

  They say this then created a fire which set off other warheads, provoking an explosion which ripped open the Kursk's twin-pressurised hulls.

• The submarine collided with the seabed during a manoeuvre, causing tanks of pressurised air inside the submarine to explode or otherwise triggering a larger explosion. A retired commander of the Black Sea Fleet, Admiral Eduard Baltin, has suggested that the accident was a result of incompetence, bad planning and bad training.

  "The Kursk is designed for the ocean, not for shallow waters. Where it was manoeuvring and where it perished is completely wild - strong currents and strong winds. You can't carry out torpedo firing there," he said.

• The submarine collided with a US or British submarine, which triggered a second explosion. Russia's navy chief Mikhail Motsak said three non-Russian submarines were in the Barents Sea at the time of the accident. "We think that it could have been a British submarine", he said. The Russian daily Sevodnya said it had evidence that the Kursk crashed into a US submersible, which then limped into a Norwegian port.

  The Pentagon has admitted that two US submarines were in the same zone, but denied they were involved in a collision.

  "We have found absolutely no indication that there has been a collision in the area," Norwegian armed forces spokesman Brigadier Kjell Grandhagen has told the BBC.

• It was sunk by an anti-ship missile fired by a Russian cruiser. According to Germany's Berliner Zeitung, quoting a report by Russia's intelligence service the FSB, the Kursk was sunk by a radar-guided Granit missile fired by the Kirov class nuclear-powered cruiser Peter the Great. The FSB, however, has denied knowledge of the report.

• The submarine hit a surface vessel, possibly a Russian ship

• It hit a mine left over from World War II.

Casualties

It is thought that whatever happened to the Kursk it happened quickly - so quickly that it could not even send out a distress call, or release an emergency beacon.

Doubts have arisen regarding the news disseminated by the Russian navy that seamen inside the vessel had been communicating with rescuers by tapping on the submarine wall.

Russian defence analyst Pavel Felgenhauer says the sounds detected were never more than a faint knocking sound coming from somewhere inside the vessel.

And a US intelligence analysis, details of which were apparently leaked to the US media, is said to indicate that no communication of any kind was heard from inside the submarine at any time after the disaster struck.