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. 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.