The Estonian authorities refused to send the flight data recorder of the helicopter to the United States because the aircraft was manufactured there, thus possibly creating a conflict of interest. The technical investigation was in fact performed in the United Kingdom. The voice recording indicated that the pilots realized that something was wrong only 35 seconds before the helicopter hit the sea, and that they did attempt to send a Mayday message.
The accident investigation board published a preliminary report on 14 September 2005, in which they ruled out most possibilities of physical damage before the helicopter hit water, including sabotage and collision with a flock of birds. As the helicopter was manufactured in the United States, the U.S. National Transportation Safety Board (NTSB) has taken part into the investigation. In November 2005, the NTSB issued an "urgent" recommendation to the FAA that it require all S-76 operators to carry out "immediate visual and laboratory examinations" of main rotor servos for plasma flakes and other contamination. The helicopter manufacturer, Sikorsky Aircraft, rejected these findings, saying it and servo manufacturer HR Textron "do not agree that the servo caused the mishap". In December 2005 the company, however, issued an all-operators letter recommending that operators carry out internal leakage tests of all S-76 main rotor servos, at the same time emphasizing that testing with an intentionally degraded servo "has not identified any safety of flight issues".
The talks between Copterline and Sikorsky Aircraft about how to divide damages arising from the crash broke down in December 2006, and Copterline sued Sikorsky Aircraft in a U.S. court in New York for damages of 60 million US dollars.
In August 2007 it was announced that the cause of the crash was a failure of the hydraulic flight control system, caused by coating flaking off from servo pistons. The loosened plasma coating flakes blocked the return valves of the servos, causing the aircraft to abruptly lose its maneuverability.[1] Copterline did not have the authority to service or even open these components. However, in the final report, Copterline was criticized for omitting a periodic leak test of the hydraulic flight control system that could have revealed the problem. The Finnish authorities were also criticized for insufficient supervision.