More than $140,000 (£105,000) worth of bitcoins paid by victims of the WannaCry ransomware outbreak have been removed from their online wallets.
It has been nearly three months since infections struck organisations worldwide, including the NHS, which faced days of disruption as a result.
The bitcoin activity was noticed by a Twitter botset up by Quartz journalist Keith Collins.
The balance of all wallets known to be associated with WannaCry is now zero.
The ransomware hit many businesses hard, quickly infecting multiple computers on corporate networks and encrypting them so they became useless.
Victims were asked to pay between $300 and $600 to get their systems back.
Back in May, many cyber-security experts and law enforcement agencies advised victims that paying the ransom would probably only encourage other cyber-criminals and not result in restored access to computers.
However, many clearly decided to take a chance.
According to bitcoin-monitoring company Elliptic, an initial portion of the WannaCry funds were moved in late July.
And at about 04:10 BST on Thursday, the vast majority were finally withdrawn in entirety.
Many watchers expect that the WannaCry bitcoins will be put through a “mixer” – in which the currency is transferred and mixed into a larger series of payments that make it much harder to track where it ends up.
But the incident has left some cyber-security experts confused.
“I have no idea why they would move that money to be honest,” said Andy Patel at F-Secure.
“I wouldn’t imagine that they are going to try and turn those bitcoins into real money. If they do, it’s going to give someone a way to track them to an actual person.”
Instead, Mr Patel told the BBC the funds could be used to pay for dark web services that might leave less of a digital paper trail.
In July, bitcoins paid as ransom following a separate attack – NotPetya – were moved from their online wallets.
By Alan Woodward, cyber-security adviser to Europol
Many people assume Bitcoin is anonymous: the online equivalent of cash. However, every transaction is completely visible to anyone who cares to look.
There are even online sites that allow you to view what is happening in the blockchain – the distributed ledger that records all bitcoin movements.
The blockchain is more like a Swiss bank account: you know the account number and which account transfers money to which other accounts, but you don’t necessarily know who stands behind that account number.
A technique called “cluster analysis” looks across all of these bitcoin addresses and attempts to find addresses that are being used by the same people.
Then, some of the other transactions in that cluster, which were not intended to be anonymous, can provide evidence of who owns those addresses.
Law enforcement agencies often use this classic approach to track criminals – the idea, of course, is: “Follow the money.”
Alan Woodward is professor of cyber-security at the University of Surrey.