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Ecologists believe the deaths were caused by the fire and release of hazardous chemicals while the Singapore-flagged X-Press Pearl burned for 12 days and sank last week off the main port in the capital Colombo.
However, government officials said these causes were "provisionally" confirmed and the investigation continues.
The fire started on the ship on May 20 and dead marine species started washing ashore days later.
A ship manifest said 81 of the ship’s nearly 1,500 containers held "dangerous" goods.
The Sri Lankan navy believes the blaze was caused by its chemical cargo, most of which was destroyed in the fire.
But debris including burned fibreglass and tons of plastic pellets have severely polluted the surrounding waters and a long stretch of the island nation’s famed beaches.
Post-mortem analysis on the carcasses is being performed at five government-run laboratories and separately by the Government Analysts Department, said an official from the wildlife department.
"Provisionally, we can say that these deaths were caused by two methods – one is due to burns from the heat and secondly due to chemicals. These are obvious," said Anil Jasinghe, secretary of the environment ministry.
He refrained from giving an exact cause, saying post-mortem analysis is "still being conducted".
Thushan Kapurusinghe, of the Turtle Conservation Project, blamed the fire and chemicals the ship carried for killing the turtles.
With more than three decades’ experience on turtle conservation, Kapurusinghe said the dead turtles had oral, cloacal and throat bleeding and "specific parts of their carapace have burns and erosion signs".
The sea off Sri Lanka and its coastline is home to five species of turtles that regularly come to lay eggs and March to June is the peak season for turtle arrivals.
Lalith Ekanayake, a marine and coastal ecologist, suspects based on the nature of the fire and amount of chemicals, "at least 400 turtles may have died and their carcasses may have sunk in the sea or drifted to the deep sea".
Sri Lanka plans to claim compensation from X-Press Feeders, the ship’s owner, and has already submitted an interim claim of 40 million dollars (£28.8 million).