At this point all Michael "knew" was that there was a traitor in his family, otherwise those shooters would never have gotten onto the grounds. He was also correctly assuming that the shooters were already dead, and that he would get no information out of them.
I'm glad you pointed that out, Apple, because it's also affirmation of how Michael knew, almost instantly, that Roth--not Pentangeli--was behind the attempt. Frankie was too unsubtle, too uncalculating, to have recruited a traitor in the family, to have found unidentifiable shooters, and to have arranged their killing--all the while getting drunk at Anthony's party and mouthing off to Michael. At that point, Michael couldn't have ruled out Neri and/or Rocco being complicit in some way, or Fredo having contributed by stupidity or inadvertence, but he knew Roth was behind it.
