Graphic: Russian sub accident

Twenty people have died on board a Russian nuclear submarine - believed to be an Akula II attack class vessel - during trials in the Sea of Japan. There were 208 people on board.

1. Personnel working in front section of submarine affected when fire extinguishers set off unexpectedly
2. Affected area may have been sealed off, trapping personnel inside. There were more staff than usual on board as submarine was undergoing tests
3. Fire extinguishers release freon gas, which displaces breathable oxygen, causing workers to suffocate
4. Twenty people die and 21 are injured