A former IT worker was sentenced to eight years in prison by the Tokyo District Court on Wednesday for sending death threats by hacking into other people's computers.

In a bizarre case, Yusuke Katayama, 32, was arrested in 2012 on charges of using a remote computer and sending a mass-killing threat to a comic book event after months of evading investigators with a series of vexing cyber-riddles. He also said he would attack an airplane and target a kindergarten attended by the grandchildren of Emperor Akihito.

"He committed the crime, and the purpose of it was for police to make wrongful arrests," said presiding judge Katsunori Ohno, adding that Katayama's actions had been "vicious."

Prosecutors had sought a jail term of 10 years for Katayama.

Katayama denied the charges and was released on bail last March as his trial proceeded. While Katayama was in court during a session, an email was sent to media organizations and lawyers, in which the unidentified sender claimed to the be "the real culprit" behind a series of email death threats in 2012 that were sent by hacking into other people's computers.

Katayama held a news conference and said he knew some people would suspect him but said he was not the culprit and called for the trial to be ended. He was released and dropped out of sight.

In the email, the sender referred to the "PC remote control virus case," and said: "I am the one responsible. It has been a while hasn't it?" The unknown sender wrote that he/she had framed Katayama for the cyberattacks. "Katayama sure has landed in a tough spot, and I felt a bit sorry for him," the sender wrote.

However, Katayama was seen wandering along the bank of the Arakawa River in Edogawa Ward and was seen to bury something. Police searched the area and found a smartphone with the message that was sent while he was in court.

Katayama was rearrested and admitted sending that email himself using a timer on a smartphone.

Earlier in the high-profile case, four individuals were mistakenly arrested during the initial investigation following an exhaustive hunt that at one stage had authorities tracking down a cat for clues.

The case proved embarrassing for the National Police Agency (NPA) after it emerged that officers had extracted "confessions" from the four people who had nothing to do with the threats.

An anonymous hacker then sent messages to newspapers and broadcasters, with the sender claiming details of a computer virus used to dispatch the threats were strapped to a cat living on an island near Tokyo.

After cracking a set of riddles, police found the cat and removed a digital memory card from its collar which revealed a message saying "a past experience in a criminal case" had caused the hacker to act.

Police analyzed the memory card and footage taken by security cameras, coming to suspect that Katayama, a resident of Tokyo, was responsible for the hacking campaign, media said.

© Japan Today/AFP