Indian navy divers have found the bodies of two of the 18 sailors who were inside a submarine when some of its weapons detonated, setting off a fire.
There has been no sign of life from the Russian-built sub since it sank on Tuesday night.
The INS Sindhurakshak, which was docked at the main naval base in Mumbai when two explosions occurred.
Admiral DK Joshi said divers had managed to open the main hatch of the vessel.
The diesel-powered submarine completed an upgrade lasting 2½ years at a shipyard in Russia a few months ago at a cost of £52 million.
The INS Sindhurakshak was one of 10 Kilo-class submarines bought from Russia between 1986 - 1990. At the time of the explosion it was said to be fully loaded with ammunition.
The BBC reports it is not clear what caused the blasts. Sabotage has not been ruled out.
Last year, India bought a Nerpa nuclear submarine on a 10-year lease from Russia at the cost of nearly $US1 billion. Russia supplies about 70% of India's military hardware.