I'm a bit surprised at your lack of imagination in this case--usually you make some imaginative, even far-fetched leaps and conclusions. :) From the beginning when you first mentioned the boss, I felt he was the most likely suspect--he was right there when she got sick at the restaurant, and also had time to administer more poison at home. As this case is about thallium poisoning, tracing how it could've been obtained is the most important piece of evidence, but you don't seem to have a whole lot of details on that; if it's tightly controlled in Japan, it shouldn't be that hard to trace the thallium. As to motives for poisoning the girl, one can imagine a whole host of plausible ones. Maybe the girl wanted more (marriage), or told him that she was pregnant (which could be a lie) and he had to bear responsibility, or they were breaking up and she found somebody else (such details may not be disclosed to others, so just 3rd parties' observation that "they were ok" after the trip doesn't mean a whole lot)--so there is a whole bunch of possible motives for a man like this philandering boss and husband who's already proven he's a bit of a bastard. To convict him, the only key is to trace the thallium back to him. Motives are no problem. Japan has a conviction rate of something like 98% (!) once a suspected is charged, and most of the time by confession; I don't know if this guy will confess (maybe not--given his irresponsible playboy lifestyle and habits), but the police likely has some strong evidence. You didn't mention whether the hospital found traces of thallium in the aunt's system (assuming it's preserved somehow, say in her blood samples or hair)--even without tracing thallium to the girl, the aunt's poisoning if proven would doom this guy. So the police has 2 bites at the apple. Finally, I wouldn't call this case to be the Japanese Zhu Ling case, which is far more tragic and protracted, a saga that garnered huge attention but little official action lasting some 30 years. The only similarity between the 2 cases is possible thallium poisoning. Zhu was more talented, still in school, and entirely innocent (not that the Japanese girl deserved to die, but she didn't seem to be sorry at all for trying to break up someone's marriage), and Zhu's parents also suffered 30 years of injustice at the hands of the CCP Chinese authorities (including the Chinghua University) and corrupt justice system that allowed the prime suspect to escape culpability because she had well-connected parents. To this day the Zhu suspect is free and living a life of luxury overseas under a different name). I can almost guarantee that the Zhu suspect will never face justice, unless it's the vigilante form (very doubtful because Chinese people are so passive and fatalistic). By comparison, probably in a few months or a year, this Japanese killer-poisoner rich playboy will most likely be convicted by the super-efficient (or even suspiciously efficient) Japanese courts. Killing 2 people in cold blood may even get him the death penalty, which Japan still has but doesn't carry out that frequently anymore.