A recent Washington Post article (July 8, 2022) on the assassination of former prime minister Shinzo Abe says
Last year, eight of the 10 shootings in Japan were related to the yakuza, according to the National Police Agency, resulting in one death and four injuries.
I'm aware the definition of "mass shooting" can change depending on the source (e.g. USA discussion ).
I'm looking for an authoritative source (Japanese government agency, academic research project, etc) that has some statistics on mass shootings for a number of years. English is preferred, but a source that google translate will work with is also acceptable.