To throw my two (or three) cents in, first of all in 2000 even in a minor city in Japan its likely that the family would have had cable internet that cutting the phone line wouldn't have interrupted.
Forgive me if I overlooked it but how is law enforcement so positive that it was the killer who briefly browsed the internet? Blood on the keyboard or something I suppose? It just jumped out at me as an odd thing to be a hard fact in the investigation.
From visualizing it as I read the posts the "disgruntled student" or "skater" hypothesis strikes me as by far the most likely. This seems like a crime committed by a very impulsive person. I initially was thinking "random psycho" until I saw the clothes but teenagers can behave similarly to psychopaths at times.
This brings my thoughts to the incongruous sand thing which they bounce hard off of. I would dive deep into the exact science of how it was determined that sand was from near Edward's AFB and assuming that science is indeed sound I would start researching any kind of US manufactured products of the time that might have contained sand. Like, I remember there used to be thick rubber balls full of sand that could be squeezed or punched or slammed and would flatten out or hold their dents. A teen with anger issues might have had such a "stress ball" that could have ruptured and left traces of sand in the bag. Skater/surfer types also often have little glass bottles of sand as trinkets/jewelry.
My final thought is on why so much time could have passed without anyone incriminating themselves or any DNA matching to have been made. I think there's a good possibility the perpetrator killed himself somewhat soon after the crime. He may have even been trying to work up the will to do it right there in the house with the bodies but he was startled by the grandma.
Thanks so much, Evilwise! If the Miyazawa's didn't have dial-up internet, that would definitely explain how the killer was able to get online (if that's indeed what he did) despite the phone line being cut.
As for how the TMPD are so sure of *what* he did on the computer, my Japanese translator gave me that information based on news media. So, there wasn't a great deal of digging to get that info. Which makes me think they'd have to be pretty certain to release that to the press?
Some say he was on the computer for 5 minutes and simply logged in and created a new folder. Which seems like a long time to do so little. Other sources say he did the same but with the added action of click on the bookmarks tab and going to a theatre website which Mikio have favourited. (We know theatre and puppetry were hobbies of Mikio).
As for the sand, unfortunately I can't go into too much detail but I can tell you that sand / soil analysis can be very, very accurate. I've seen some of the testing and it's honestly pretty incredible. However, whether the TMPD had access to this at the time of the murders, I don't know. Unfortunately, it's not too clear at what point they know about the sand. To further complicate matters, the TMPD themselves never mention Edwards AB, only ever major newspapers (but it seems HIGHLY unlikely that they would run this detail with insider information from the police).
But what we do know for sure is that the sand was found in the hip bag of the killer. And the bag was manufactured in Osaka one or two years before the murders.
I have been to Edwards AFB (or as far in as you can get without clearance) and I have been to the surrounding towns. Nothing much of anything is manufactured there, so far as I could see -- not that me eyeballing the area on several occasions is conclusive of anything, of course.
And as for the killer taking his own life--yes, that's very possible. Certainly, few better hiding places than being dead. Especially in a country where cremation is the norm. That's an interesting point about working up the will, too. That might explain why he would stay so long in the house--I'd never considered it that way.
