The navy’s chief said a search party had recovered fragments from the KRI Nanggala 402 including items from inside the vessel, whose oxygen reserves were already believed to have run out.
Warships, planes and hundreds of military personnel have been searching for the stricken vessel. Authorities had said the German-built craft was equipped with enough oxygen for only three days after losing power.
That deadline passed early yesterday.
“We have raised the status from submiss to subsunk,” navy chief Yudo Margono told reporters, adding that the retrieved items could not have come from another vessel.
“(The items) would not have come outside the submarine if there was no external pressure or without damage to its torpedo launcher.”
Navy officials displayed several items including a piece of a torpedo and a bottle of grease used to lubricate a submarine’s periscope.
They also found a prayer mat used by Muslims.
The submarine - one of five in Indonesia’s fleet - disappeared early Wednesday during live torpedo training exercises off the Indonesian holiday island.
An oil spill spotted where the submarine was thought to have submerged pointed to possible fuel-tank damage, fanning fears of a deadly disaster.
There were concerns that the submarine could have been crushed by water pressure if it sank to depths reaching 700 metres - well below what it was built to withstand.
The vessel was scheduled to conduct the training exercises when it asked for permission to dive. It lost contact shortly after.
Authorities have not offered possible explanations for the submarine’s sudden disappearance or commented on questions about whether the decades-old vessel was overloaded.
The military has said the submarine, delivered to Indonesia in 1981, was seaworthy.
Singapore and Malaysia, as well as the United States and Australia, were among nations helping in the hunt with nearly two dozen ships deployed to scour a search zone covering about 10 square nautical miles (34 square kilometres).
Australia’s HMAS Ballarat arrived yesterday with a US P-8 Poseidon aircraft also helping to look for the craft.
Singapore’s MV Swift Rescue - a submarine rescue vessel - was expected later yesterday.
Indonesia’s military said earlier it had picked up signs of an object with high magnetism at a depth of between 50 and 100 metres, fanning hopes of finding the submarine.
But yesterday’s announcement means the Southeast Asian archipelago joins a list of countries struck by fatal submarine accidents.
Among the worst was the 2000 sinking of the Kursk, the pride of Russia’s Northern Fleet.
That submarine was on manoeuvres in the Barents Sea when it sank with the loss of all 118 aboard. An inquiry found a torpedo had exploded, detonating all the others.
Most of its crew died instantly but some survived for several days before suffocating.
In 2003, 70 Chinese naval officers and crew were killed, apparently suffocated, in an accident on a Ming-class submarine during exercises in 2003.
Five years later, 20 people were killed by poisonous gas when a fire extinguishing system was accidentally activated on a Russian submarine being tested in the Sea of Japan.
And in 2018, authorities found the wreckage of an Argentine submarine that had gone missing a year earlier with 44 sailors aboard.