A computer technician at Mossack Fonseca has been arrested on suspicion of removing large amounts of data from the company, the BBC understands.
The employee was arrested at the law firm’s Geneva offices after the company provided information to prosecutors.
The employee is accused by the firm of theft of data, unauthorised access and breach of trust.
Thousands of documents were leaked from Mossack Fonseca in April, revealing systematic offshore tax evasion.
Mossack Fonseca says it has not broken any laws or destroyed any documents, and says all its operations were legal.
It is not known at this stage whether the alleged data theft cited by prosecutors this week is linked to the leak in April, dubbed the “Panama Papers”.
The Panama Papers were investigated for months by hundreds of investigative journalists, including staff from the BBC.
The Geneva prosecutor’s office began a criminal inquiry in April into the Panama revelations.
The anonymous source behind the Panama Papers spoke out in May, offering to help law authorities make prosecutions in return for immunity.
In a 1,800-word statement, “John Doe” said he had never worked for a spy agency or a government and cited “income equality” as a motive for the leak.
It is not known whether the source had any contact with law enforcement officials after his statement.
Panama Papers – tax havens of the rich and powerful exposed
Eleven million documents held by the Panama-based law firm Mossack Fonseca were passed to German newspaper Sueddeutsche Zeitung, which then shared them with the International Consortium of Investigative Journalists. BBC Panorama and UK newspaper The Guardian were among 107 media organisations in 76 countries which analysed the documents. The BBC does not know the identity of the source
- The documents show how the company helped clients launder money, dodge sanctions and evade tax
- Mossack Fonseca says it has operated beyond reproach for 40 years and never been accused or charged with criminal wrongdoing
- Read more: How assets are hidden and taxes evaded