But on 9 October, the real perpetrator sent an email to a lawyer - Yoji Ochiai - and local media, explaining how he or she made those threats by taking control of innocent internet users' computers with a virus.
His or her purpose, as stated in the email to Ochiai, was "to expose the police and prosecutors' abomination".
And in a way, it did. It raised the question - why did the innocent people confess to a crime that they didn't commit? What kind of pressure were they put under?
"I was surprised to have received the email but I wasn't surprised that the innocent people confessed," says Ochiai.
There have been a number of wrongful convictions in the past, he says.
"But unlike other cases, the fact that these cyber threat incidents happened to ordinary people who were just using the internet raised the fear that it could have happened to anyone," he adds.
But what makes Japanese suspects eager to confess - even to a crime that they didn't commit?
Lawyer Yoji Ochiai thinks it has something to do with the Japanese psyche.
"People traditionally thought that they shouldn't stand up against authorities so criminals confessed quite easily," he says.
"But in the 21st Century, more people - guilty or not - are exercising their rights and wouldn't simply obey and confess."
"The authorities still try to extract confessions using the same methods and that's why they end up pressuring suspects to confess which may have resulted in untruthful confessions," he added.