evidence vs confession.
Unfortunately, in Japan the police are more interested in confessions than in actually securing any evidence. The lengths they will go to in order to secure "confessions" is relatively well documented (just one relatively thorough article on the subject here: http://www.iht.com/articles/2007/05/07/news/japan.php)
So to me, and to anyone who knows about the Japanese legal system, a "confession" isn't really worth that much. I'd rather the police actually have evidence. If they have that, I'd be more confident that they got the right person.