Bitcoin is an experimental digital currency that has gained popularity worldwide / Getty Images

In the last week, the equivalent of more than £60m has been drained from accounts

Just like any other kind of cash, the online currency Bitcoin can be stolen. And over the past week, as many as 96,000 Bitcoins - equivalent to more than £60m - have been purloined from the users of the shady online bazaar, Sheep Marketplace. The virtual heist ranks alongside the biggest real-life robberies in history, and some amateur online sleuths are convinced it was an inside job.

In October, the notorious Silk Road website - an online trading space for guns, drugs and other black market materials - was shut down. In its absence, Sheep Marketplace became one of the most popular narcotics exchanges on the so-called “deep web”. Its customers paid for their illicit purchases using Bitcoin, which offers near-anonymity to its users.

Last week, according to a report by The New Statesman, a person or persons began draining the Bitcoin accounts of the site's buyers and vendors, while hacking those accounts to make them appear untouched. Only over the weekend did Sheep Marketplace administrators uncover the theft and promptly shut the site down, by which time the thieves had made off with up to 96,000 Bitcoins - each of which is worth at least £650 at the current exchange rate.

Several swindled Sheep users crowd-sourced an investigation on reddit, where some alleged the theft was a scam by the owner of the site, known as “Tomas”. The identity of the perpetrators, not to mention the precise amounts involved, remain a mystery. However, due to a quirk of the Bitcoin transaction process, it is possible to follow the trail of the stolen cash. Though Bitcoin purchases can be pseudonymous, each transaction is visible to the public.

As the thieves attempt to launder their haul by “tumbling” it into ever smaller amounts and parcelling them into multiple accounts, reddit detectives tracked the stolen funds using Blockchain, a site which monitors Bitcoin transactions. While the sleuths remain in hot pursuit, the thieves have no way to convert the Bitcoins into cash without it being traced to their real-world bank accounts.

In one reddit post, a user called silkroadreloaded2 wrote: “All day, we've been chasing the scoundrel with our stolen bitcoins through the blockchain. Around lunchtime (UK), I was chasing him across the roof of a moving train, (metaphorically). I was less than 20 minutes, or 2 blockchain confirmations, behind 'Tomas'.”

While online drug transactions may be safer than those on street corners, the Sheep theft demonstrates the inherent risks of the anonymous online trade in illicit goods. Following the October arrest of Ross Ulbricht, the alleged owner of Silk Road, the administrator of a similar site, Project Black Flag, closed up shop and fled the web, taking his clients' Bitcoins with him. Last year, hackers stole 24,000 Bitcoins (then worth approximately £150,000) from the exchange site BitFloor.