TIGARAS, Indonesia -- Indonesia's military chief said Thursday that specialist navy equipment will be deployed to pinpoint a ferry that sank in the crater lake of an ancient supervolcano, offering a glimmer of hope to distraught relatives that the bodies of more than 180 people presumed drowned will be recovered.
Only 18 people, including the captain, have been rescued and four confirmed dead since the ferry, massively overcrowded with passengers and motorcycles, sank early Monday evening in waters that officials say are up to 500 meters (1,640 feet) deep.
As the search on Lake Toba continued Thursday, hundreds of people waiting at a small port for news of missing relatives performed an hour of mass Muslim and Christian prayers that were punctuated by sobs of grief.
Each time a search and rescue vessel docked, relatives ran toward it, only to turn away with faces contorted in disappointment or crying out the names of loved ones when it became clear that no family member had been found.
There was mounting anger that hardly any bodies have been recovered. TV broadcast footage of one anguished man angrily confronting a police officer about the fruitless search effort. The officer defended himself by saying he hadn't slept for three days.
The disaster, likely Indonesia's worst sinking in more than a decade, has prompted President Joko "Jokowi" Widodo to call for an overhaul of safety standards for passenger boats. Ferries are an important means of transportation in the nation of more than 17,000 islands, which cover an area that would stretch from New York to London.
Muhammad Syaugi, head of the national search and rescue agency, said 184 people are missing, revised down from Wednesday's figure of 192 after further checks showed some victims were counted twice. The search area has been doubled to 12 kilometers (7.4 miles) around the approximate location of the sinking, he said.
In Tigaras, on the island of Sumatra, military chief Hadi Tjahjanto told reporters that officials believe many victims are trapped inside the sunken ferry. Specialist deep water equipment was to be sent from Jakarta on Thursday afternoon, he said.
"Our navy equipment that can reach up to a depth of 600 meters will be able to determine the exact location of the ship," he said. "When we can determine where the ship is, we will determine the technique of how to retrieve the victims."
Divers have only been able to reach depths of 50 meters (164 feet) and an underwater drone deployed in the search has a maximum operating depth of 200 meters (656 feet).
Agus Darojat, the police chief of Samosir island on Lake Toba, told MetroTV that police had detained the rescued ferry captain, who was also the boat's owner.
"We focused first on providing health care for him to recover, because his condition is still unstable and traumatized," Darojat said. "He's still unable to answer our questions, could not focus to explain nor listen to our questions. It's obvious he is in shock and traumatized."
National Police Chief Tito Karnavian said local transportation officials must also be held responsible.
About a thousand people crowded Tigaras pier on Thursday, including several hundred relatives of victims, some weeping uncontrollably, others waiting in strained silence.
Cellphone video taken from another ferry that attempted to rescue people after the sinking has spread widely online and on television. The video shows dozens of people struggling in rough waters and crying for help while several of them try to swim for an orange lifesaver apparently thrown from the ferry.
Maruddin Siagian, waiting with other family members for news of his younger brother, said they're haunted by those images.
"We hope the government will never stop the search, never stop, until all the victims are found," Siagian said.
"We're tormented waiting for news about him, without any certainty like this. Especially our mother," he said. "But we're determined to keep waiting until the body of our brother is found. For us, his body is precious, we want to bury him properly for his soul to be peaceful, and so we are too."
The 1,145-square kilometer (440-square mile) Lake Toba, formed from the caldera of an ancient supervolcano, is a popular destination on the island of Sumatra and one of 10 stunning natural attractions in Indonesia that the government aims to develop as magnets for international and local tourists.
The doomed ferry didn't have a passenger manifest, causing confusion about how many people were on board. On Wednesday, after the full scale of the tragedy emerged, Transport Minister Budi Karya Sumadi said the boat had a passenger capacity of 43 - making it five times over capacity - and was equipped with only 45 life jackets.
Dudon Satiaputra, a former head of the national's police's forensic center, said lake beds are typically covered with thick mud or sediment and this could hamper the search for bodies.
"What I know is there is lot of mud in lakes so drowned corpses usually sink into the mud, making them difficult to float," he said. "It is different with the sea, where the bloated body will float quickly to the surface."
Sarmini Nasution said she will not leave Tigaras until her 32-year-old son is found.
She said he drowned while trying to swim for a life jacket thrown from another ferry. The two friends he was traveling with reached it and survived.
"We've been waiting for three days without certainty," she said, weeping. "I'll wait until my son is found. I will not go home before carrying his body."
Karmini reported from Jakarta, Indonesia. AP writer Ali Kotarumalos contributed from Jakarta.