Maybe the authorities do know and it's simply not published/released? After all, assuming he shot himself and the gun is right there, it seems fairly reasonable to tentatively presume it's self inflicted.
I'm unable to think of anyway in which someone can shoot themselves and not have it described as "self inflicted", even an accidental self shooting would be described by some in either form.
However, in general, with a gunshot wound and and a gun discarded right there some people might not assume that it was self inflicted and perhaps check for handedness, gunshot angle, residue, etc. before leaping to any premature assumptions or conclusions.
Presuming and Assuming when it comes to a death investigation is often the start of how innocent people go to jail