The official says the navy believes there is no way anyone could have survived the intensity of the blasts and fire Wednesday.
The official spoke on condition of anonymity because no bodies have yet been recovered.
Officials had said earlier that there had been no contact with the sailors since the explosions, which lit up the sky above the Mumbai navy base where the vessel was docked. They said navy divers had opened one of its hatches but had not yet been able to enter the submarine because it was dark and full of muddy water.
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