Comments from feedback form - "Sorry but it wasn't an "intern..."
Sorry but it wasn't an "international" investigation. They comprised only potential allies of South-Korea (one European, Americans and the rest South-Koreans), in particular, there where no Chinese. Furthermore, access to information was under "discretion" of South-Korea.
But "international", like it or not, contains connotations of impartiality and consensus. An "international" effort was also mounted against the United Kingdom and the other members of the Allied Forces during World War 2, but that's generally not the phrasing we use to describe what the Axis and their allies were doing.
Sweden, Australia, Britain and the United States and Canada (whether military staff or civilian experts) according to Wikipedia. [1]
Whether international or not, these exercises are something occurs every year to every other year depending on U.S. military commitments elsewhere. Threatening nuclear response to routine exercises is an escalation on the part of North Korea, regardless of who sank the ship.