It is impossible for one person to have killed the whole family. The lady and the daughter could have locked themselves up in their room while the killer was strangling their son or fighting the man at the stairs. It is unlikely this was carried out by one single person. The fact the killer stayed in the house for a few hours proves he had company. If robbery was the motive, they could have done the same thing to the house next door where only an old lady lived. Mikko had argued with a group of motorcycle gang few days before the murder. It is possible a few of the biker guys were so angered that they decided to teach him a lesson.
Thanks for the post, ck932! However, very respectfully I would disagree with your point that it's
impossible for one person to kill the whole family -- and for a few reasons:
1) Firstly, the sheer amount of evidence. There was blood, hair, fingerprints, excrement, saliva, and clothes left behind from one man -- the killer. After 22 years, it's hard to believe that the Tokyo MPD would hide the existence of
other assailants in the house. And if they're not hiding that, then how is it possible for the killer to leave behind so much evidence, while at the same time his co-conspirators leave behind nothing?
2) Yasuko and Niina were upstairs in the attic at the time of the attack. This was accessed by a folding ladder. We don't know if this was lockable or if there was any way to prevent someone downstairs pulling it down, even if they had tried to pull it up. That's making the assumption that they had any idea what was happening downstairs. Likely that they heard noises. But did they have any conception of what was happening? I think if you factor in the sheer human shock of seeing a masked man with a knife in your house in the middle of the night, coming out of your kid's bedroom -- it's entirely conceivable that a normal salaryman would be overwhelmed by that, even if they fought (like it seems Mikio did). The same goes for Yasuko. Why would she assume this stranger is here to murder them? Why wouldn't she assume it's a robbery?
3) Why does the fact the killer stayed in the house prove he had company?
4) I can't get into too much detail here but it's not correct that an old lady lived next door alone. There was actually another family directly next door. Including Yasuko's sister and mother (the old lady).
5) I've read the theory that Mikio may have had some kind of disagreement with a Bōsōzoku gang. To my knowledge, it's never been proven or established that this ever actually happened. Unless you have a source for it that I haven't seen?
6) Finally, we simply don't know the motive. As you say, if it WAS robbery, then the killer did several things that make zero sense from start to finish. Then again, it's also a fact that he did steal money from the Miyazawa family and that he did go through their belongings.
The problem with this case, and part of the reason I've been so taken with it for so many years, is that when you hold any single piece of it up to the light to make sense of it, you immediately find contradictions. It's an enigma. That's part of the reason why I had to make a podcast about the case (which will be out in a few weeks!)