Thank you, Mini, for your ideas again. To follow on from Ninja's reply:
Yes, the TMPD identified the knife quickly and tried to track down purchases of it locally. They found one made the day before at a nearby supermarket. For whatever reason, it took them more than 20 years to find this person. This is what fuelled the rumours of there being CCTV. It turns out that, if there ever was CCTV, it was of this individual. The Chief confirmed to me that this man on camera had nothing to do with the murders. So, another dead end.
The killer indeed brought the sushi knife with him and when it broke switched to the family's carving knife.
RE: unidentified substances in the killer's hip bag or clothes. There are conflicting reports. You can find the actual TMPD information appeal here where they list the information they have released to the public:
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Robbery and Quadruple Murder of a Family in Kami-soshigaya 3-chome（上祖師谷三丁目一家4人強盗殺人事件） 警視庁
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https://www.keishicho.metro.tokyo.lg.jp/multilingual/english/safe_society/wanted/seijo.files/eng.pdf
Not much relating to sand or substances. Some have suggested a kind of 'glassy' substance was found relating to grip tape on skateboards. Others say there was theatrical paint like you would find in theatres. Others say leaves or traces of bird droppings. I wouldn't be confident enough to rely on any of that. Seeing as the killer didn't leave his shoes at the scene and foot print evidence (in blood) is not an exact science, I'm not sure how anyone could know how an unidentified substance was found on his shoes. The TMPD say nothing of this, at any rate.
RE: the killer being a teenager on a rampage the hated females. The police seem to think he was young, between 15-25, I believe. So that's a solid guess. However, did he hate females? It's guess-work. He killed two males and two females. Then again, he displayed the most raw violence against Yasuko. Was this due to her gender? Again, we don't know. It's also possible that it was simply that he knew all threats were removed by that point in the sequence of murders so he was free to let go. As for rampage, I don't buy it. The most wanted fugitive in Japan and yet none of his other rampage crimes turn up before or after? That just doesn't make sense.
RE: the killer being a skateboarder. This is covered at length in the podcast. Basically, the link only really exists because of the proximity of the skatepark and the unverified possibility of the grip tape in his bag. I can tell you the TMPD worked this angle very hard. They really went after skaters. But it's been two decades and nothing. It seems unlikely to me that a guy who would move in a closed circle like skaters would be completely unknown to them and that in 22 years not one person would come forward. I think it's more likely he moved alone.
RE: a disgruntled solider with mental issues. It's as good a guess as any. My preferred theory is that he is the son of a USAF member at the nearby Yokota US air base or that he's simply dead. But there is zero in the crime scene that suggests any kind of military training. As for his mental state, again, total speculation. The fact he stayed so long at the crime scene suggests a disconnect with reality. But then his actions after the murder are mostly logical -- he patches himself up, doesn't seem to panic, seems to be looking for something. If he knew or thought he got away it, maybe it made more sense to stay there seeing as he was hurt. Still, we don't actually definitively know how long he stayed in the house beyond a 1-2 hour window. Put simply, he slaughtered an entire family, so he's unlikely to be Mr. Normality. But without knowing his reasons it's impossible to say.
RE: a profile, there was an Asahi TV documentary with an FBI profiler who said he believes the killer acted out of resentment. Ann Irie promptly sued the channel and the documentary went away. Some have said copies are still out there. I actually spoke to the profiler in question and he was super cagey about the whole thing. A few podcasts go into the killer's likely mental state, who he is, so forth. Frankly, I call BS. We know his actions but not his motivation. There isn't even a reasonable guess to his motivations. The Chief assured me nothing sexual happened in the house. And the only thing taken from the house was a small amount of money (though the killer left a substantial amount of money on the desk by the computer he used, which he must have seen). Certainly, the profilers I spoke to in the making of the podcast were reticent to start making guesses.
RE: the ice cream. His saliva was found on the container and the police worked out he ate it with one hand (by squeezing it out) due to the injury on the other. I don't understand why he would bother with ice cream on his wound when he used the family's first aid kit / Yasuko's sanitary pads.
As Ninja said, the killer used his own shoes. I assume he was a different size to Mikio. We don't know what size Mikio was but it's reasonable to conclude they didn't match seeing as the killer took other items of clothing (though not on the lower half of the body). This, I presume, was due to the fact that the killer's white shirt was completely covered in blood. His jacket too, possibly. I don't think the killer would go to the trouble of trying to 'throw off' the police when he was leaving his own blood, excrement, clothes, and murder weapon behind. As for what he wore being odd. I mean on the one hand it was winter and it was cold. The only thing that strikes me as a little strange is the hip bag. But then again, if he used it for books and pens (as the TMPD believe he did), then maybe not. Still, there was no class at that time of year. So, it's almost certain he brought it with him for the knife. But yes, this case is odd from start to finish!
