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A quirk in PricewaterhouseCoopers' IT settings made it a cinch for a whistle-blower to obtain thousands of documents cataloging secret tax deals between big clients and Luxembourg, an auditor at the firm said on the first day of a trial that may determine the role of informers in global tax scandals.

Former PwC staffer Antoine Deltour, 30, was able to access files containing some 45,000 pages about tax arrangements that were meant to be secret just by taking advantage of a quirk in the way files were stored on the firm's computers, according to Anita Bouvy, an internal auditor. She said an in-depth internal probe revealed the large-scale alleged theft.

"There was a technical peculiarity by Microsoft that allowed access to such restricted files" indirectly via a folder linked to a scanner, Bouvy told the three-judge panel at the Luxembourg court. "This was not known to us at the time" and "this would not be possible anymore today."

Global attention to a series of mass leaks exposing questionable tax deals this month switched to the revelations by a group of investigative journalists of millions of pages of financial records in the so-called Panama Papers. Luxembourg is still picking up the pieces after the same group in 2014 published the LuxLeaks papers, hitting the country like a "tsunami" with the revelation about some 28,000 documents naming more than 340 companies, including Walt Disney Co., Microsoft Corp.'s Skype and PepsiCo Inc.

The leaks in November 2014 prompted calls for the resignation of Jean-Claude Juncker, the country's former prime minister who is now president of the European Commission.

Another high-profile whistle-blower, Herve Falciani, a former computer worker at HSBC Holdings Plc's Geneva private bank, was found guilty of corporate espionage in absentia by a Swiss court in 2015 and given a five-year prison sentence for trying to sell client data.

"Shutting off whistle-blowers, without whom we would not even be talking about cracking down on tax avoidance, would be immensely harmful to the public campaign for tax justice," said Fabio de Masi, a left-wing German member of the European Parliament who will testify at the trial in support of the accused. "If they are found guilty, and potentially even sentenced to prison, this could have enormous effects in Luxembourg and beyond."

Deltour is among three men to face charges from theft to laundering and violating business secrecy, in the six-day trial that started in Luxembourg Tuesday.

He and Raphael Halet, 39, who both worked for PwC before, risk as long as five years in jail for allegedly stealing hundreds of private documents cataloging tax pacts between big companies and Luxembourg authorities. Journalist Edouard Perrin, 44, another French national, is the third man to stand trial, charged with being Halet's accomplice and urging the ex-PwC staffer in his search for secret files at the firm.

The ripple effects of LuxLeaks sped beyond Luxembourg, causing European Union regulators to expand a tax subsidy probe and propose new laws to fight corporate tax dodging, while EU lawmakers created a special committee to probe fiscal deals across the 28-nation bloc.

EU Competition Commissioner Margrethe Vestager was invited but declined to testify at the trial, her spokesman Ricardo Cardoso said Tuesday by e-mail. She said in her response that "the LuxLeaks files have given access to a remarkable quantity of tax rulings and constitute an interesting source of market information."

Bouvy, the only witness to testify Tuesday, was asked repeatedly how it was possible that PwC employees could access these confidential files if they didn't have the clearance. The documents concerned sensitive tax arrangements for big clients of the firm that are not even shared with clients, Bouvy said.

Halet faces the same charges as Deltour, but for stealing and using 16 documents containing tax returns from PwC.

When asked if PwC appreciated Deltour, Bouvy said he was known to perform better than the average. PwC uncovered the fraud only in 2012, after several of its clients had been contacted by journalists, and after seeing a couple of TV programs that made it clear so-called tax rulings had gotten into the hands of journalists, she said.

"After more research we saw that only one person, Antoine Deltour, had access to the documents," Bouvy said. A frustrated and disillusioned Deltour decided to quit in 2010 and took the documents with him on his eve of his departure from PwC in October that year.

Another internal investigation conducted "under the greatest discretion and confidentiality" helped unveil how Halet had worked with the journalist Perrin to access several confidential tax files, and then attached them to an e-mail account set up together with Perrin. Halet was dismissed from PwC at the end of 2014, after the LuxLeaks revelations.