A thumb drive containing hundreds of thousands of documents had landed at the front desk of French daily Le Monde in early 2014, days after the paper published an expose about people using Swiss bank accounts to hide money from the tax man.
At the weekend, the International Consortium of Investigative Journalists (ICIJ) published the data, claiming banking giant HSBC's Swiss division helped clients in more than 200 countries evade taxes on accounts containing $119 billion.
The source of the documents had seen Le Monde's tax story, written by Gérard Davet and Fabrice Lhomme, and wanted to "give them a hand," Lhomme told France Inter radio, refusing to divulge the source.
The documents were originally provided to the French government by whistleblower Hervé Falciani, a former IT employee of the HSBC private bank in Geneva.
On the thumb drive were "all of HSBC's clients from the end of 2006 to March 2007, customers from around the globe, more than 100,000 people and corporate entities," he said.
Also included was internal bank correspondence.
"One night we printed the files. The printer went wild. We piled up hundreds of pages," Lhomme told AFP, who added it left him and his colleague "snowed under."
So the reporters decided to call in the Washington-based ICIJ in order to share the data with other outlets all while retaining editorial control of the investigation.
Lhomme said they were aiming to expose the names of public figures, but only after giving them a chance to explain themselves.
Thus 154 reporters from nearly 50 countries and 55 outlets, including Britain's The Guardian and Swiss newspapers such as Tages Anzeiger and Le Temps, got to work.
In September 2014 about 40 of the journalists gathered and met for nine hours at Le Monde's office in Paris to give the investigation a boost, Lhomme said.
The journalist said he made several trips to the United States to work with ICIJ "because there are things you can't do by secure phone lines and restricted email. It was too risky."
When the collective could not meet face-to-face they used an encrypted forum to exchange information.
"We devoted a manic attention to checking everything several times, to ensuring the numbers weren't doctored," Lhomme said.
The documents showed that HSBC opened Swiss accounts for international criminals, businessmen, politicians and celebrities, according to the ICIJ.
"We went a lot further, a lot faster than the authorities," Lhomme said.
"We learned things that neither law enforcement nor tax authorities had.
"The investigation is global, there are no borders."