The Tokyo District Court on Wednesday handed down an eight-year prison term to a hacker who hijacked computers and used them to send malicious threats, leading to the erroneous arrest of the devices’ unsuspecting owners.

The case, which dates back to 2012, raised questions about policing practices after four people were wrongfully arrested in connection with the cyberthreats.

Two of those arrested even gave false confessions for issuing the messages, which included bomb threats and a vow to go on a mass-killing spree.

The police later apologized for their handling of the case, which critics say also underscored fundamental shortcomings in their skill at investigating cybercrime.

Yusuke Katayama, 32, a former information technology professional, “took full advantage of his expertise” to issue online threats of violence in a way that “framed innocent people while dodging arrest himself,” said the presiding judge, Katsunori Ono.

Katayama conducted “thorough preparation” before committing the crimes and used “cunning” tactics to mislead investigators.

“It was a very malicious kind of cybercrime,” Ono said.

Katayama has yet to decide whether to appeal the ruling, his lawyer, Hiroshi Sato, told reporters on Wednesday.

The hacker had hijacked a number of personal computers belonging to other people and used them to send 10 threats or false alarms remotely. He stood accused of five charges including intimidation and business obstruction.

One threat concerned a bomb aboard a New York-bound Japan Airlines airplane in August 2012. The flight was forced to return to Narita Airport, and the carrier incurred ¥9.75 million in costs.

Between June and September 2012, Katayama also sent death threats to a variety of schools, local municipalities and event organizers, warning of a planned “massacre” and “indiscriminate killing” of children and others.

The threats resulted in four wrongful arrests in Tokyo, Osaka, Fukuoka and Mie prefectures.

All of those arrested initially denied any wrongdoing, but two of them were later allegedly coerced by interrogators into making false confessions.

Katayama was previously arrested in 2005 over an online death threat made against employees of the Avex Inc. music label.

The ruling on Wednesday said Katayama had perpetrated the 2012 threats in part to retaliate against police authorities and to besmirch their reputation. It said his motive had been “selfish” and this meant no extenuating circumstances could be taken into account in the ruling.

Although prosecutors had sought a 10-year prison term for Katayama, the eight-year prison term reflected a complete failure on the part of the defense, Sato said after the ruling.

Sato had been playing down Katayama’s culpability with regard to the four arrests, saying those were the result of police failings.

But Wednesday’s ruling held that Katayama, not the police, were primarily accountable for the wrongful arrests. Framing innocent people, it said, had been the defendant’s intention all along and the arrests were the direct result of Katayama’s meticulous planning.

Sato had also been asking for clemency on the grounds that Katayama had suffered from developmental disorders. The court rejected this argument due to lack of evidence.

The nearly yearlong trial took a dramatic twist in May when the defendant suddenly abandoned his not-guilty claim and admitted he had perpetrated the crimes. Before that, a confident Katayama had maintained his innocence and had even been released on bail.

The turnaround came when surveillance officers spotted Katayama burying a smartphone on a riverbank in Tokyo to conceal evidence he had used to concoct his alibi. He was arrested hours later.

That act constituted an “unprecedentedly proactive, malicious” attempt to destroy evidence for a person on bail, and reflects his “persistent” determination to evade criminal responsibility no matter what, the ruling said.

Shortly before his arrest, Katayama told Sato that he is a “psychopath,” and claimed that he was able to deceive people without feeling the slightest remorse, the lawyer quoted him saying at that time.

“(Katayama’s) deeds, in a way, are a form of pleasure-seeking crime, one that’s symptomatic of this Internet age, where people somehow frustrated with their lives no longer stab or punch somebody directly to vent their anger,” Sato said.

“They instead turn to the world of cyberspace, where they deceive strangers and take grim satisfaction in watching them suffer.”