I agree. Though there is the possibility that the DNA tests suggesting a European ancestor were misinterpreted by Japanese researchers, I agree with you that any mixture of ethnicity would be Asian "heavy".
The knife salesman evidently did not notice any strong indicators that the perpetrator was mixed ethnicity, nor an obvious foreign accent, speaking in broken Japanese etc.
As a side note, previous generation(s) mixed marriages and out of wedlock births from the US military presence and the former British, French, Dutch colonial presence can easily lead to people who today are citizens of Asian countries and are 75% or 88% Asian and only 25% to 12% European (or less)- thus European features would not be very noticeable.
All MOO: Probably, "the colonial way" would explain some Western European Y-cariotypes among the Asian men. The colonial way is always about the land, so foreign men move in, kill the local males, settle in and impregnate the local women. (Men don't bring mitochondrial DNA, mitochondria comes from maternal egg. Men bring Ys.) Some prevalence of European mitochondrial DNA among Asians, as I suspect, might be related in time to the slave markets, especially in Constantinople, where female abductees from Europe would be sold, probably, all over the world. So could European mitochondrial DNA end up among Asians? Surely, in most places, with a very few exceptions, tribal belonging was defined by the father. And, there potentially could always exist an influx of foreign women to all countries, whether more or less, depended on the history and the coastal line, I think.
Mitochondrial DNA is foreign for human eukaryotic cells. Our mitochondria, the cell powerhouses, are the descendants of the first aerobic bacteria residing within first prokaryotic cells. (They came and stayed, and with them, the cells got aerobic glycolysis, a humongous evolutionary step. Endosymbiosis.) However - mitochondria, the foreign bacteria, have own DNA, totally different from the cell's nuclear DNA, and while it mutates, the mutations accrue over thousands of years. Mitichondrial DNA comes from direct maternal line, but when that "initial" European mother appeared in the Asian population, is impossible to tell. Or maybe some Asian merchant impregnated a foreign woman during his travels, could happen, too.
I can think for ages how and where my Russian ancestor's maternal line acquired their uber-rare mito DNA. It is more frequent in Iran, but maybe Iranians got it from another group, too. So imagine that in 2001, something happened in Helsinki, and Finnish police got my relative's DNA. With genetic knowledge of 2001, they would see: Y is of N1C1 haplotype. "Karelian, that's us". And then, this super rare "Iranian" mito DNA. What would they assume? Oh, the guy's dad was probably a Finn, and his mother, from Iran (and with more recent immigration, they'd say, could be Afghani or even some Pakistani tribe). That the guy was born in Central Russia and likely, inherited his Y from some small Northern Siberian ethnicity and his mito - who knows, nomades living in Azov steppes would be my guess, would totally escape them.
Now, hopefully, the Japanese police knows a little bit more and probably performed rudimentary autosomal analysis of the perp's DNA. But if not, they may be looking at the results of very old genetic expertise and their hands might be tied. (But it doesn't mean the perp is necessarily Korean. We don't have his big Y results).
So: modern DNA analysis should be done. Also, we might be totally missing some middle situation. Not only there were, and are, many Koreans living in Japan, in difficult circumstances, what about someone from the base impregnating a local woman in, say, 1984? She could be Japanese, or not at all, Tokio is a megapolis. Or what about some American couple adopting a Chinese/Korean/any Asian boy? The parents may be great people and explain their kid's behavior by adoption issues.
