After the first explosion the crew of the Japanese ship report seeing 'flying objects' coming in towards them, and the manager of the company is adamant from listening to their testimony that the subsequent explosion was 'definitely not a mine or a torpedo', it says.
In Japanese:
https://headlines.yahoo.co.jp/hl?a=2...00080-jij-soci
And from NHK, in English until they take it down. A 'projectile' is the translation here:
https://www3.nhk.or.jp/nhkworld/en/news/20190614_31/