France has sent a nuclear submarine to help find the Air France jet that crashed into the Atlantic on Monday, killing all 228 people on board.

The sonar surveillance equipment could help find the Airbus's black boxes, which are designed to emit a signal for 30 days. The search for the plane's wreckage is being hampered by worsening weather. The French authorities have now formally launched an investigation into the crash. French media are speculating that the pilots, facing extreme turbulence, may have been flying too slowly and too low.