(CNN)It was the biggest story of their careers, and investigative journalists Frederik Obermaier and Bastian Obermayer knew the forces they were up against.
How nail polish helped keep the 'Panama Papers' secret
"The most important rule for everybody was to really be careful who you talk to," Bastian told CNN's Fred Pleitgen on Monday.
Last year, they had received a tip from an anonymous source -- to this day, even they don't know his or her name -- asking if they were "interested in data."
What they would receive was the biggest leak of information in history -- terabytes -- orders of magnitude more than WikiLeaks, Snowden's leaks, or anything else.
It immediately became apparent that what would become the "Panama Papers" had the potential to embarrass -- or worse, implicate -- world leaders and internationally sanctioned individuals, by linking them to offshore bank accounts that allegedly shielded wealth from taxes or authorities investigating wrongdoing.
So that's where the nail polish came in.
"That was the part where my girlfriend really laughed at me, when I bought that nail polish," Frederik said.
There was a serious purpose.
They had already set up a special room at the offices of their newspaper, the German daily Süddeutsche Zeitung. Only a few people had access, and the computers within it had never been connected to the internet.
They wanted to be absolutely sure that the data trove they sat on would be safe from prying eyes, or prying hands, wishing to make it all go away.
"It was very important to keep it secret."
So they went one step further and used the glitter nail polish (color unknown) to paint over the screws of their computer cases -- if anyone tampered with them, they hoped, the polish would be cracked and they would know something was up.
"You know, in Germany, it's not that dangerous," said Bastian. "But still, we had those thoughts once in a while, yes. Yes, we did."
"We were more worried about the safety of our colleagues in Russia and in Africa and in the Middle East."
It is important to remember, said Frederik, that the documents are "not only about tax avoidance and tax evasion."
"For example, there's one case, when we see that they speak about the cousin of Bashar al-Assad, who was sanctioned in 2008 by the U.S. And they realized that he is using some of the companies they set up but they kept him for a customer."
That man is Rami Makhlouf, a powerful man in Syrian politics and business.
In 2008, the U.S. Treasury Department put him under sanction, saying he "has used intimidation and his close ties to the Asad [sic] regime to obtain improper business advantages at the expense of ordinary Syrians."
A full three years later, just weeks before the uprising and crackdown that would lead to the Syrian civil war, an email contained in the data, from an employee at Mossack Fonseca, made clear the law firm was happy to continue doing business with the man. (The term "Panama Papers" arose as a reference to the country where Mossack Fonseca is headquartered).
"'As far as I can see, there are allegations (rumors), but not any facts or pending investigations or indictments against these persons," the firm's employee wrote.
It would take until that fall, according to the Guardian, for the firm to break ties with Makhlouf.
"We saw documents where they call him -- 'Oh, he is the cousin of Bashar al-Assad,'" said Frederick.
"There are documents where they realize that has claimed or alleged to be one of the financiers of the Syrian regime. But it seems that they didn't really care at the beginning."
In a taped interview released just after the leak went public, the law firm's co-founder, Ramon Fonseca, said: "We do not participate in the activities of the company, nor do we have any responsibility over what the company does."
"Remember that due diligence" -- looking into whether any clients or business partners were questionable, like Rami Makhlouf -- "is something recent," Fonseca said in Spanish. "Ten years ago, there was no due diligence. The term was not even known."
CNN reached out to Makhlouf through his company, Syriatel, but has yet to receive a response.
Fonseca told the Financial Times he does not expect the leak to lead to "one single legal case," but Frederick said the firm's co-founder may not want to speak so definitively.
Being associated with an offshore account -- as the journalists say the Panama Papers show Makhlouf was -- is not in and of itself illegal. And the journalists do not speculate what the money in Makhlouf's accounts would have been used for.
But running afoul of American sanctions, or sanctions elsewhere, in dealing with a so-called "designated" person or entity could have serious implications.
"Although the company is headquartered in Panama, they have offices in the U.S. So there would be a possibility for U.S. authorities to get after Mossack Fonseca.
The journalists realized from the start the leak was "going to be a big story," but it was right before they were about to take the story public that they realized just how huge it would be.
"It was at a time when we at Süddeutsche Zeitung internally tested our Web page," said Frederick.
"Our technicians told us that they had to test it some minutes before the official publication date."
"And then we then saw Edward Snowden tweeting about this publication -- that was a surreal moment."
"And for us it was, like, whoa, was it really just Edward Snowden tweeting about the project we worked the last year on?"
"We set up our English Web page of Süddeutsche and in the first 24 hours there [were] more English-speaking readers than German readers, and that was the first time in the history of this newspaper."