An IT worker at Mossak Fonseca, the company at the heart of the "Panama Papers" leak, was arrested Wednesday in Geneva.
The arrest was made as part of the investigation into the leak, which saw 11.5 million documents from the law firm leaked to the German newspaper Süddeutsche Zeitung.
The documents detailed thousands of offshore companies set up by Mossak Fonseca on behalf of rich clients, sometimes for the purpose of tax avoidance.
The identity of the worker has not been released, and the Süddeutsche Zeitung reporter who led a year-long investigation into the documents said he did not believe the arrested worker was his source.
Still, a lawyer for Mossak Fonseca told Swiss newspaper Le Temps that the millions of documents were accessed and downloaded via the arrested worker's computer.
No other details were released. The newspaper quoted a lawyer for the arrested worker denying all charges.
The leak caused embarrassment for many, including politicians who had used the law firm to establish overseas companies to hold assets. Among the casualties was Iceland's prime minister, who resigned in April.