When an accident occurs it is investigated by the government with jurisdiction over the area where a plane goes down. The sole objective of the investigation of an accident or incident is the prevention of accidents and incidents. It is not the purpose to apportion blame or liability.
Investigations are conducted in accordance with the international standards and recommended practices as described in ICAO Annex 13 - Aircraft accident and incident investigation.
While the initial field phase of an accident investigation can be concluded within weeks or even days, the investigators' final report and recommendations often take years to complete. The final report consist of factual information about the accident, an analysis, conclusions (probable cause) and also includes safety recommendations.