At the Wednesday news conference in Moscow, the commander-in-chief also said that cause of the catastrophe has now been established. As fuel is used, large cavities are formed in the fuel tanks, explained the general. At an altitude exceeding four kilometers, the low pressure may flatten the fuel tanks. To avoid this, the cavities are filled with liquid nitrogen in flight. In the crashed bomber, the system of filling the fuel tanks with liquid nitrogen failed and the tanks collapsed at the high altitude, said the general. There was no guilt of the crew in the crash, stressed the commander-in-chief of the Russian Air Force.