Svartholm Warg is suspected of having hacked into the IT security company Logica, which supplies several government agencies with information from the tax authority's public register, in March this year.
“This is a serious incident. We have never experienced something like this before,” said Anders Sandell, head of security at the company, to Expressen at the time of the hacker attack. .
The tax agency confirmed to the paper at the time that a number of personal identity numbers had been accessed in the breach, of which about a thousand were of people with protected identities.
“This is a terrible failure for the tax agency. People with protected identities are generally crime victims, abused and persecuted women, witnesses under threat, judges, police officers and journalists. Their lives could depend on not being detected,” said a source close to the investigation to the paper around the time of the incident.
Svartholm Warg, who denies his involvement in the hacker attack, was detained when his plane touched down on Swedish soil on Tuesday. He had been deported from his home in Cambodia and sent back to Sweden where he is already facing a one-year prison sentence for promoting copyright infringement.
Cambodia arrested Warg at Sweden's behest and the decision to deport him comes despite there being no extradition treaty between the two nations.
Shortly after Svartholm Warg's arrest at Arlanda airport, reports emerged that he was wanted in an investigation into the hacker attack against the tax agency from earlier this spring.
"During a preliminary investigation concerning computer hacking at the company Logica and the Swedish Tax Agency during the spring, suspicions were directed at one person, Gottfrid Svartholm Warg," the Prosecution Authority wrote in a statement Tuesday.
The agency added, however, that Svartholm Warg's deportation from the country on Monday was due to an arrest warrant issued for him in relation to his conviction in the Pirate Bay trial.
Svartholm Warg, and his fellow co-founders Fredrik Neij and Peter Sunde, as well as financier Carl Lundström, were all convicted in 2009 of facilitating copyright infringement and ordered to pay 46 million kronor ($6.9 million) in damages to the music and movie industry.
While Neij, Sunde, and Lundström all had their 12-month sentences reduced to between four and 10 months on appeal in late 2010, Warg did not attend his appeal hearing and his one-year sentence was upheld.