I'm Maeve McLanagan and this is The Tip-Off, the podcast where we take you behind the scenes of some of the UK's best investigative journalism.

Today's story is a relay race.

In reality, it involved more than 300 reporters across six continents.

But we're going to focus on just three journalists.

My name is Bastian Obermeier and I'm the deputy head of the investigative department Department of Suddeutsche Zeitung.

We start with Bastien.

He works for the German paper Suddeutsche Zeitung.

And in the past, he's written about illicit arms deals,

sexual abuse by priests and Nazi war criminals.

But in early 2015, he was about to start down another path, one that would lead to the biggest

story of his career.

And it started with just eight words on a screen.

Hello, this is John Doe.

Interested in data?

Well, on the first moment, I was a little curious and interested.

That was a message Bastian received one day. A message from an anonymous source. It was quickly followed by

more.

My life is in danger. We will only chat over encrypted files. No meeting ever.

Intrigued, Bastian started to message the source regularly.

I agreed to receive more documents.

In the next messages that I received from the source,

I also got some more and more documents later. Then, you know, after some days, when I received more and

...