NTSB(National Transportation Safety Board) in the United States.
Air crash investigations are handled by the country where the crash occured.
The main purpose is to explain accidents and provide safety recommendations.
After a crash?
In the US, a ”go-team” of technical experts often immediately travel to the place where the crash occured. The area is treated very similar as to a crime scene. The area is secured, and all evidence are documented, mapped and collected. Most important is to recover the cockpit voice recorder and the flight data recorder. All factors – mechanical, operational, and human performance are taken into account.
Sometimes the governments spends whatever it takes to find out the cause of a crash, both in manpower and money. The 1996 explosion that brought down TWA Flight 800 is a great example of that. A small ”army” of people were involved in that investigation.