A man accused of leaking information exposing Luxembourg's tax deals with multinational companies only had access to the confidential documents because of a security glitch, a court heard on the opening day of the "LuxLeaks" trial on Tuesday.
Antoine Deltour, a French citizen and former employee of accounting firm PwC, is accused of passing data on PwC clients to journalist Edouard Perrin for a French television broadcast made in 2012.
Prosecutors say this data and material allegedly supplied by a second former PwC employee, Raphael Halet, was used in the 'LuxLeaks' revelations of November 2014 by the International Consortium of Investigative Journalists.
The charges against the three Frenchmen range from violating secrecy laws to theft and IT fraud. Deltour's lawyer said his client did not set out to find the documents, but only came across them by chance.
French Finance Minister Michel Sapin expressed "solidarity" with Deltour, saying the 30-year-old was defending the public's interest.
"It's thanks to him that we've been able to put an end to the opacity that prevented European countries from fully knowing the tax status of a number of large companies in Luxembourg," Sapin told lawmakers in the National Assembly.
Sapin has instructed France's ambassador in Luxembourg to assist Deltour during his trial if needed.
The LuxLeaks reports prompted accusations that Luxembourg had conspired with multinational companies to form tax arrangements that deprived other European Union states of revenue.
The leaked documents showed that companies such as PepsiCo , AIG and Deutsche Bank secured deals from Luxembourg to slash their tax bills.
The Grand Duchy says other countries have similar arrangements, and has offered to share details of the tax deals with other states.
During Tuesday's hearing, a PwC expert said Deltour copied 45,000 pages of documents that he was able to access because of a glitch in the company's servers, which had since been fixed.
"He found them while looking for training documents," the lawyer, Philippe Penning, told reporters.
The trial, criticised by some as aiming to gag whistleblowers seeking to uncover corporate tax avoidance, attracted dozens of chanting protesters outside the courthouse.
Deltour faces up to five years in prison and fines of 1.25 million euros ($1.42 million) if found guilty.
"The message is to say, today citizens want more fiscal justice," said Frenchman Francois Thierry, wearing an "I support you Antoine" T-shirt.