A fishing trawler sank in freezing waters off Russia's Far East on Thursday, killing at least 56 people and setting off a frantic search for survivors that led to dozens being rescued alive. In the worst such accident in recent Russian history, some 132 people were on board the Dalny Vostok when it went down at around 6:30 am (2030 GMT on Wednesday) in the Sea of Okhotsk off the Kamchatka peninsula, possibly after a collision. A local governor speculated that the vessel had been overloaded. Sixty-three people were rescued while 13 were still missing and rescuers halted active search operations as night fell. The emergency ministry put the death toll at 56. Seventy-eight of those on board were Russians, mostly from the Far Eastern region, and there were 42 Myanmar nationals, five from Vanuatu, three Latvians and four Ukrainians. World leaders expressed their condolences to President Vladimir Putin, including German Chancellor Angela Merkel, who said she had learned of the sinking "with dismay" and wanted to express her "sincere sympathy." All the survivors were taken aboard a ship. A helicopter with a medic on board made a failed attempt in two-metre (six-foot) waves to airlift those seriously injured to hospital in the city of Magadan, 250 kilometres (155 miles) to the north. It was set to make a fresh attempt early Friday. "Nine people are in grave condition, some are unconscious and some in a state of shock," said Tatyana Yukhmanova, a spokeswoman for the emergency ministry in the Kamchatka region. "Although they were wearing wetsuits, they suffered hypothermia because the water temperature is around zero degrees Celsius (32 degrees Fahrenheit)," she told AFP. The uninjured survivors and the victims' bodies will be transported to the port of Korsakov on the island of Sakhalin, but the journey will take three days, the state RIA-Novosti agency said. - 'Collision' possible - The Soviet-built trawler dates from 1989 and is a 5,700-ton, 104-metre long factory ship is designed to be at sea for several months at a time. It was purchased only last year by Magellan and before that was operated by a Baltic marine fishing company and was called Stende. Speculation mounted on the possible reasons for the accident. The ship apparently sank so quickly that the captain did not manage to make a distress call. Russian investigators launched a probe into a possible violation of safety measures. They seized documentation from the ship's owners and conducted searches in the vessel's home port in Nevelsk as well as in the far eastern port city of Vladivostok from where the ship departed on January 3. "The investigation plans to question members of the crew and company management shortly," investigators said. "Currently the investigation is looking into a possible collision with an obstacle damaging the ship's hull near the machine room as the most likely reason for the ship's rapid sinking," they added. But as many as 10 possible scenarios are being considered, Emergencies Minister Vladimir Puchkov said. Sakhalin's acting governor Oleg Kozhemyako said the ship was in adequate condition and likely capsized due to weight distribution violations. "Most likely loading and ship balancing rules were violated. Either they had the lower decks empty or they didn't load the ballast on time," he said in televised remarks. "The vessel capsized." - Illegal crew? - One expert questioned whether many of those on board were working illegally. Pyotr Osichansky, who heads the Vladivostok-based centre for investigations of naval accidents, told Channel One television the ship had just over 60 people on board when it departed Vladivostok. "That means that most likely (other) workers were taken on board in (South Korean port) Busan and are therefore illegal," he said. Kozhemyako, the Sakhalin governor, said the foreigners on board were not wearing lifejackets or wetsuits, raising questions as to whether there had only been enough safety equipment for the official crew. Many fishing vessels trawl the seas of the Russian Far East. Last December, a South Korean fishing boat sank in the Bering Sea, between the eastern coast of Russia and Alaska, with 60 aboard, seven of whom were rescued.