[Reports on the shooting Friday in Munich that left nine dead say that the gunman likely obtained his weapons online illegally via the "dark net." The following Die Welt article earlier this month covers another German case, in the city of Stuttgart, and the issues around black market activity on encrypted websites.]

BERLIN — The three young men knew how to make lots of money. All they needed was a small workshop at grandma's, access to the Internet and a few guns that fired blanks.

One of them was a toolmaker. He would be the one to turn the blank-firing guns into real weapons, mostly Walther PK380 semi-automatic pistols. The second had the funds to buy the material needed for the operation. The third would help assemble the weapons.

And then there's Darknet, a kind of parallel Internet in which you can remain anonymous or use a fake identity. That would be the perfect place to sell their homemade weapons. Later, they thought, they might even graduate to big guns like the Zastava M70 or AK-47 assault rifle.

That's the story the Stuttgart district attorney pieced together after months of investigation. The suspects are German nationals — one aged 24 and the two others aged 28 — who now face charges for trading illegal weapons online.

Although the investigating officers do not know how the men managed to import four illegal assault rifles from China and Yugoslavia, they are sure that the youngest of the three was the one to sell the weapons online, including ammunition, for a total of 11,200 euros. The two others were supposed to launder the money through a petrol station.

The case is now to be tried at the Stuttgart district court and it throws light on the dark underbelly of the Internet.

You can get anything and everything illegal on the internet from computer viruses to drugs and weapons. An online black market has been founded by criminals for whom it is easy to remain undetected in the so-called Darknet. This corner of the internet can only be reached through anonymous services such as Tor.

Tor users worldwide — Source: Stefano.desabbata

Criminals of the 21st century do not have to rob banks or go to clubs to sell their drugs or drive across the border to sell their weapons. They only have to smuggle malware onto someone's computer or sell their wares at the black market of the digital underground.

The Internet is an uneven playing field. Investigators are fighting ghosts. Most of the time, the ghosts win. But sometimes, things turn out differently and investigators are able to take down a dealer or even an entire network.

But a dealer is only caught if he or she was careless and left visible traces. This was apparently what recently happened with the three Germans. The district attorney general was investigating several men who apparently dealt with weapons, weapon parts and ammunition. A raid in autumn of 2015 led to the discovery of emails on one of the defendant's phones. These emails suggested that he bought assault rifles, converted them and sent them to a delivery address in Paris. The timing was conspicuous: it happened shortly before the Paris attacks on Nov. 13, 2015.

The men were then suspected to have knowingly or unknowingly supplied the weapons for these attacks. But it soon emerged that evidence supporting these suspicions was not sufficient. The district attorney has stressed this point repeatedly. Despite the fact that the youngest of the defendants sent the assault rifles to a "non-existing drop site" in Paris it was never established whether the assault rifles ever reached Paris or, indeed, ever left Germany.

The three suspects have been in remand for months now and are expected to receive prison sentences of up to five years.