We may never know, as that squad probably 'doesn't exist'. They are almost certainly a tier 1 group.
Not likely, given the fact that we know that
something necessarily killed him, and given that the US has gone so far to announce that they killed him on several different channels on several different media...
Unless the way in which he was killed was "politically untoward" then there's no reason to even lie about
how he was killed.
EDIT: Rather, it's possible that a tier I unit killed him, but I don't think there's a reason to think that the gov't would deny that a tier I unit killed him if such a unit
were responsible.