Akerosid
There is, it is Annex 13 to the Chicago Convention.
It covers the investigation of both accidents and incidents. As you say, responsibility for the investigation of an accidnet rests with the "state of occurrence". The state of occurrence has to notify the accident to the states of registry, design and operator, all of who are entitled to participate in the accident investigation.
A copy of Annex 13 can be found at:
http://www.sq006.gov.sg/about/ICAO%20Annex%2013.pdf