The Sanchi was carrying 136,000 tons -- around one million barrels -- of oil from Iran to South Korea when it collided with the Hong Kong-registered CF Crystal freighter in the East China Sea on January 7.

The tanker was carrying condensate, an ultra-refined, highly volatile form of ultra light crude oil used to make products such as jet fuel.

Potential environmental disaster

Environmentalists and officials are worried the oil on board and fuel used to power the massive vessel could harm aquatic life for decades.

"The critical thing is to understand that when we put hydrocarbons into the oceans through events like this, it's going to affect a wide range of animals," said Jessica Meeuwig, a professor of biological sciences at the University of Western Australia.

The collision site was within the Zhoushan shipping ground, considered to be one of the richest fishing grounds in China, Ma Jun, one of China's leading environmentalists said.

"We still need to keep an eye on how these contaminants might be carried by the ocean flow to have the impact on the fishing ground."

Condensate oil's high volatility means it evaporates more easily than heavier crudes. Some of the oil on board the Sanchi -- though it's unclear how much -- was likely consumed in the fiery blaze after the crash, environmentalists say.

Three bodies were recovered over the week and just hours before the Sanchi sank to the ocean floor.

Iran's Special Task Force spokesman Mohammad Rastad said testimony from rescued crew on the CF-Crystal, as well as from rescue workers, suggested the Sanchi's crew likely died early on from a massive blast and the ensuing release of toxic gas, state media reported.