The German government and security agencies have been accused of not taking internet security seriously, following a huge data breach that affected hundreds of politicians and celebrities.

Joachim Herrmann, interior minister for the southern state of Bavaria, said he was appalled at the way the federal government and information security agency, the BSI, was handling the scandal, the biggest data leak in German history, after it was revealed it had dismissed a breach in December as one-off incident.

“I was astonished at the way they communicated this, it was bewildering,” he told the tabloid Bild.

Herrmann said he believed the perpetrator behind the hack was an individual and not a foreign government, as was initially feared, with many pointing the finger at Russia.

A 19-year-old German man was being questioned by police on Monday, over his alleged involvement with the hacker believed to be responsible. Police raided the teenager’s house in the town of Heilbronn in south-west Germany on Sunday and took away the contents of rubbish bins and computer equipment.

Identified only as Jan S, he has denied being the main perpetrator behind the leaks but claims to know “Orbit”, the hacker who has claimed responsibility via Twitter.

Jan S, who works in the IT industry, told the state broadcaster ARD he had been questioned “for several hours”. He is so far being treated only as a witness to the security breach, having allegedly been in communication with Orbit.

It was revealed on Friday that the BSI was investigating a data leak affecting many prominent politicians, including the chancellor, Angela Merkel, and the president, Frank-Walter Steinmeier. The information, drip-fed on Twitter throughout December, included mobile phone numbers, credit card details, contact information and family photographs. Celebrities and journalists were also affected.

Herrmann insisted the BSI should be forced to reveal what it knew when, and why it failed to crack down on the breach at the earliest opportunity.

The federal interior minister, Horst Seehofer, has come under attack for failing to address the issue publicly and for not providing reassurance to ordinary Germans about the safety of their data.

His parliamentary state secretary, Stephan Mayer, told German media on Monday that Seehofer would give MPs a detailed assessment of the cyber-attack in a special meeting of the Bundestag’s interior committee on Thursday.

On his Twitter account, Jan S said he had been in touch with the hacker known as Orbit for years via an encrypted messenger service. He said Orbit had sent him an email shortly after the publication of the hacked data, telling him he was planning to destroy his computer so he could not be traced. Jan S said the alleged hacker had since deleted his account with the messenger service.

The hack is likely to increase Germans’ comparatively high degree of scepticism towards social media, experts say. Among the more prominent victims of the hack who said they would drastically alter their use of social media was Robert Habeck, co-leader of the Green party, who said he would delete his Facebook and Twitter accounts.

He described the panic he felt on realising that large amounts of data from his accounts, including family photographs, had been hacked, but said he also regretted the manner in which he had frequently adopted a polemical style to further his arguments.

Habeck said social media had encouraged him to be “more aggressive, louder, more polemical and pointed – and at a speed in which it’s hard to allow any room for reflection,” he said.