As the “full and meticulous” investigation ordered by Russia’s president Dmitry Medvedev into the recent submarine accident which killed 20 sailors goes on, the ambiguity about what really happened and what caused the tragedy is starting to disappear.

The victims on the Nerpa nuclear-powered submarine died of asphyxiation. The vessel’s fire extinguishing system set off all of a sudden and released toxic freon gas which is used to suppress fire. The accident raises a lot of questions about Russia’s military professionalism and capability.

This tragic incident was the worst suffered by the Russian Navy since the explosion of the Kursk back in 200. As much as 118 sailors were killed when the Kursk exploded under water.

Besides the 20 dead, 21 other people are ill after the incident that occurred on Saturday when the submarine was conducting sea trials in the Sea of Japan. The ill were hospitalized and their radiation levels are reportedly “normal,” as a Russian Navy spokesman put it.

There were many civilians aboard the submarine when the incident occurred and this may have increased the death toll. Only three seamen were killed in the accident which happened in the front part of the submarine. The nuclear reactor is in the stern of the vessel and it wasn’t affected. There were no radiation leaks, Russian Pacific Fleet spokesman Igor Dygalo said.

The sea trials were being conducted in anticipation of leasing the submarine to India, who sent personnel to Vladivostok to train on board the vessel, according to the website Indian Defence.

One of the problems aboard the submarine was overcrowding. There were 208 people aboard, while the vessel’s capacity was of 73. There were 81 seamen aboard. The inexperience of the personnel was another factor that led to the fatal accident.

According to navy captain Gennady Illarionov, many civilians probably weren’t properly equipped and they most likely did not know how to use the breathing apparatus, state news agency RIA Novotny reported.