"He works on the night shift and is probably asleep," says an elderly woman as she walks past us and opens the door to her ground floor flat.

She's referring to her neighbour across the landing whose front door has empty beer bottles outside, and a large bag of garden fertiliser on the doormat. We knock on the door. No response. We wait a few moments and chap again. Still nothing.

The Ferret is in Edinburgh on a dank winter's day trying to track down a 'company' linked to an international money laundering scandal - involving an estimated £770m. We're in Muirhouse to be precise, a housing estate in Scotland's capital made famous by its favourite son, Irvine Welsh, who immortalised the working class area in his 1993 novel, Trainspotting.

Royston Mains Street is our second stop today. We drove here after doorstepping another nondescript tenement flat, a few minutes drive away in Granton Crescent, also in Muirhouse. There was no-one there either, nor any sign of the 'company' we're investigating which is registered as a Scottish Limited Partnership (SLP), a legal entity under UK law.

There are 1,156 SLPs registered at this address.

SLPs date back to the Limited Partnerships Act of 1907. They are used by thousands of legitimate businesses - but some have been used by money launderers, tax evaders and fraudsters, prompting calls for reform to laws to crackdown on criminality. SLPs are easy to set up and can hold assets and enter into contracts, which helps to obscure ownership and facilitate activities like money laundering.

They've been of interest to The Ferret for some years now. In 2023, for example, we found that only three out of 631 SLPs had been formed by residents of Scotland. Two years prior, we revealed that a couple of Edinburgh addresses had been cited as the official headquarters for businesses that use opaque financial structures linked to alleged money laundering and tax abuse.

Now, as part of our finance project, The Money Trail, SLPs are in sharp focus again.

In Edinburgh, the SLP we're trying to track down is called Octmedia and its registered address is the top floor flat we visited in Granton Terrace. We want to speak to the people behind this SLP because it was used to launder money generated in an international scam co-run by a man who called himself the "King of Fraud".

According to court documents seen by The Ferret, Octmedia was used to hoard much of the ill-gotten gains of an internet fraud scheme. The operation saw fraudsters set up advertising companies to sell online ads, and charge clients proportionate to the number of views the ads received. They also generated millions of artificial views from 650,000 IP addresses that were leased.

The conmen infected 1.7 million computers with malware, which covertly accessed the internet to generate ad views. Their complex scheme was powered by 1,900 computers that were rented from a Texas data centre and used remotely to post ads onto 5,000 spoof websites.

In November 2021, Aleksandr Zhukov, the self-described "King of Fraud" who helped set up two ad networks in 2014, was found guilty and sentenced to 10 years in prison in the US. Two other individuals pleaded guilty. While the court indictment refers to revenues of £28m generated by the fraud, some estimates put the total at £770m.

Our investigation found that Octmedia was founded in 2016 by two firms based in the Seychelles - a so-called 'secrecy jurisdiction' where company owners can eschew public registers. The following year, new UK regulations made it mandatory for SLPs to disclose their owners.

But Octmedia simply named another SLP, Johnson and Brothers LP, as its owner - a legal loophole used to avoid disclosing the names of individuals. Johnson and Brothers - also registered at the Granton Terrace flat as a SLP - was itself owned by a pair of Seychelles companies during the fraud operation.

However, leaked Pandora Papers files, obtained by the International Consortium of Investigative Journalists and ...