Who investigates a plane crash?
The United Nations’ specialized agency for civil aviation called The International Civil Aviation Organisation (ICAO) dictates the rules on this, spelled out in its Annex 13. They set out that the country where the crash occurred leads the safety investigation. It also says that the countries where the plane is manufactured, owned and operated should be invited, as well as where the majority of the deceased are from. That can encompass many different countries, agencies and private companies. But if the cause is terrorism, then it turns into a criminal investigation and the rules change.
Notice Russia launched a criminal investigation?