NOT since 1993 had a Japanese journalist been investigated in South Korea. But this time it was not classified military intelligence that was allegedly divulged—but hearsay. On October 8th prosecutors charged Tatsuya Kato, until recently the Seoul bureau chief of the Sankei Shimbun, a Japanese right-wing daily, with defaming the South Korean president, Park Geun-hye. Mr Kato is currently banned from leaving the country.

The source of the upset is an article which the Sankei published online on August 3rd. It speculated on the whereabouts of Ms Park on the day a ferry sank in April, claiming 304 lives. Many blame the deaths on a botched rescue operation. Rumours have spread that at the time Ms Park was out of contact for seven hours. Citing the Chosun Ilbo, South Korea’s biggest daily, that mentioned but rather ridiculed the gossip, as well as reports circulating among stockbroking houses, Mr Kato suggested she was rumoured to have vanished for a tryst with a divorced man. The president’s office staunchly denies this.

Some Japanese say the case has targeted Mr Kato because the Sankei is the standard-bearer of Japan’s irksome historical revisionism. It has for years campaigned to reverse an apology from Japan over the forcing of Korean women into wartime brothels. Dokdo Saranghoe, a South Korean civic group that defends islets claimed by Japan as South Korean territory, was one of three groups that lodged a complaint about the article on grounds of libel.

The affair will do little to help strained bilateral relations. Few South Koreans have any sympathy for the Sankei, but that is precisely why Mr Kato is “the perfect scapegoat”, says Oh Chang-ik of Citizens’ Solidarity for Human Rights, a liberal lobby in South Korea. He says the case is an attempt to cow South Korea’s domestic press. Prosecutors have already searched the home of a reporter at NewsPro, a South Korean outlet that translates foreign news, including articles from the Sankei.

Defamation lawsuits have been used before by the country’s presidents, conservative and liberal. In 2011 a host on a South Korean podcast that lampooned the then president, Lee Myung-bak, was sentenced to a year in prison for spreading false rumours about him, alleging past involvement in stock fraud. In 2003, when he was president, the late Roh Moo-hyun filed a lawsuit against four South Korean dailies for linking him to dodgy property deals.

To some, this is heavy-handed. South Korea enjoys a thriving civil society and competitive elections. Yet its libel law is strict. Truth is no defence against spending time in prison (punitive damages are unknown in the South Korean system). Instead, the public interest needs to be proved. Both the Sankei and Reporters without Borders, a Paris-based watchdog, say Mr Kato’s article met that standard.

Last month Ms Park said insulting the leader had “crossed the line”. Prosecutors swiftly set up a team to monitor the web for falsehoods or defamations. For Cho Guk of Seoul National University this is a depressing return to tendencies associated with the dictatorship of Ms Park’s late father, Park Chung-hee, a military strongman. Two crimes were notorious then: criticising the leader and spreading false rumours.

The crackdown on rumours has prompted some 1m South Koreans to ditch local chat apps within a week—including KakaoTalk, the country’s biggest—for Telegram, an encrypted service based in Berlin. This week KakaoTalk said it would stop honouring warrants from prosecutors (who have denied they monitor private conversations). Reporters without Borders ranks the level of surveillance of South Korea’s internet as similar to that of Egypt and Thailand. Last year censors deleted or blocked over 80,000 web pages, for pornography or gambling, but North Korean sites, along with those of sympathisers of North Korea, are also blocked under the National Security Law, a cold-war legacy. That law was once abused to silence critics, and it continues to rankle. But now the defamation law has become the government’s tool of choice, says Mr Cho.

Last year the UN’s free-speech envoy said many South Korean suits are filed to punish statements that are true or in the public interest. As the Sankei case rumbles on, South Korean media with reservations about Mr Kato’s harsh treatment may censor themselves. In private, journalists admit that writing anything positive about Japan is almost impossible in the current climate.

President Park says that by insulting her, the likes of Mr Kato insult her nation. Her nation might wonder whether the greater insult was to its hard-won democracy.