US naval experts say the most likely cause of the Kursk disaster was an on-board explosion of a weapon which prompted an even larger blast that sank the nuclear submarine.

The Pentagon has been trying to analyse what happened to the Russian vessel using data obtained from an intelligence gathering ship and two US nuclear submarines that were monitoring the Russian exercise.

The US still insists that there was no collision and that it was the on-board explosion that sent the Kursk to the bottom.

Relatives of the dead want answers from the government

US experts cannot be certain about the exact sequence of events that precipitated the Kursk tragedy, but they can make a pretty fair guess based on their own data, which appears to be backed up by that from a Norwegian intelligence gathering ship and seismic monitors that recorded the Kursk's sinking.

Every submarine - indeed every surface vessel -has its own distictive sonar signature.

The US Navy knows the Kursk quite well, having monitored it last summer in the Mediterranean when it was shadowing a US carrier battle-group.

Pentagon experts believe that there is nothing to suggest that poor handling of the submarine contributed to its loss.

The sonar and seismic data back up earlier western theories, indicating two on-board explosions; one of moderate size, followed about two minutes later by a devastating blast that sent the submarine to the bottom.

Volatile fuel

The best US theory is that the first explosion involved fuel from a torpedo or a long-range anti-shipping missile carried by the Kursk.

The Pentagon says poor handling was not to blame

This then created a fire which set off other warheads, provoking an explosion which ripped open the boat's twin-pressurised hulls.

Nobody knows exactly what weapons the Kursk was carrying or whether the initial accident came when handling the weapons or during the preparations for a test-firing.

Some Russian reports speak of the Kursk carrying a new type of torpedo - unpopular among submariners - because of its highly volatile fuel.

Many existing Russian torpedoes are believed to be powered by hydrogen peroxide which can also be highly volatile especially in confined spaces.

These are of course just theories - only the Russians themselves may be able to say exactly what happened - and that may require an extensive underwater survey of the wreck, and perhaps even its eventual salvage.