The wooden boat, which sank Monday on Sumatra's Lake Toba, a popular tourist destination, was thought to be operating illegally, with no manifest or passenger tickets.
![Linn of Tummel Waterfall [UKNIP]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Linn of Tummel Waterfall [UKNIP]")
The situation has sparked confusion -- and different estimates from various agencies -- about the number of people on board.
Indonesia's disaster agency, separate to the search-and-rescue body, originally said some 80 people were on the boat along with dozens of motorcycles when it overturned and sank.
Police said in a statement Wednesday there were as many as 178 people missing, which if confirmed would make it one of Indonesia's worst maritime disasters.
The estimates have been based on local people reporting missing relatives who they believed could have been on the ship.
"But many people got on the boat without a ticket so it's unclear how many were on board,"Muhammad Syaugi, the head of the search-and-rescue agency, told AFP.
So far, at least three bodies have been found and another 18 people rescued, according to the local disaster agency, as the search turned to recovering victims -- including those still trapped inside the sunken boat.
The search-and-rescue agency said search operations would continue for at least a week given the size of the enormous body of water, which fills the crater of a supervolcano that is believed to have erupted tens of thousands of years ago.
It is one of the deepest lakes in the world and covers some 1,145 square kilometres (440 square miles).
'We just want to see his body'
On Wednesday the size of the search was increased to around 400 personnel.
"We are looking to search an area as deep as 400 metres, but we haven't found anything yet because the area is very large," Syaugi said.
Images from the scene Wednesday showed rescuers covering up the bloated body of a woman who had washed ashore.
It was not clear if any foreigners were on board the ferry or what caused the disaster.
"We'll be here until they find my brother's body," said Nurhayati, who was among the hundreds of grief-stricken locals waiting by the shore for news about missing loved ones, and like many Indonesians goes by one name.
"We just want to see his body and take him with us."
Survivor accounts said the boat started shaking as it struggled to navigate strong winds and high waves about halfway into the 40-minute trip from an island in the middle of the lake to shore.
Muslim-majority Indonesia has been celebrating the Islamic festival of Eid since Friday and millions go on holiday during the festivities, with Lake Toba a key destination.
The deadly disaster came days after more than a dozen people were killed in an unrelated maritime accident that underscored Indonesia's woeful boat safety record.
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![The woman was recorded, among others, while bathing [BI]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The woman was recorded, among others, while bathing [BI]")
