The K-152 Nerpa was carrying many more people than usual

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