Are they still traceable after tumbling?
Sometimes, yes...
"Address deanonymization using these methods can be thwarted by sending bitcoins through so-called mixers or tumblers, which take a set of bitcoins and returns another set of the same value (minus a processing fee) with different addresses and transaction histories, thus effectively “laundering” the coins. But these services come with serious caveats. Users must hand over control of their bitcoins and trust the service to return them. Transaction graph analysis can identify use of a mixing service and flag the user as potentially suspicious. Mixers do not work well for very large sums, unless others with similarly large sums happen to be mixing their bitcoins at the same time. Some mixing services do not work as advertised and can be reverse-engineered. Services that operate legally must keep detailed records of how the coins were mixed, which could later be hacked or subpoenaed. And the new bitcoins received might themselves be tainted by illegal activity."
Source: https://coincenter.org/entry/how-anonymous-is-bitcoin
'But even mixing has weaknesses that forensic investigators can exploit. Soon after Silk Road shut down, someone with administrative access to one of the newly emerging black markets walked away with 90,000 Bitcoins from user escrow accounts. The thief tried to use a mixing service to launder the money, but wasn’t patient enough to hide the tracks, Meiklejohn says. “It’s difficult to push large amounts of Bitcoin through mixing services secretly. It’s extremely noticeable no matter how you do it.” Thomas Jiikovský, the man under investigation by Czech police, is suspected to be the thief in question.'
Source:
http://www.sciencemag.org/news/2016/03/why-criminals-cant-hide-behind-bitcoin
'Another technique known as ‘tumbling’ would allow the hackers to pool their ill-begotten bitcoins with other people's coins.
'In a bitcoin tumbling service, coins from different sources are mixed together and then re-disbursed. Conceivably, the hackers could repeatedly mix their coins until the coins were diluted enough to throw law officials off their path.
'But Ethan Heilman, the Boston University researcher behind TumbleBit, a proposed bitcoin tumbler, indicated that mixing bitcoin is risky business, especially when dealing with larger sums of money. As he pointed out, one of the problems the hackers may run into is finding a large enough number of bitcoins to adequately mix with.
'"Even if they mix the coins such that they will be hard to follow, if the WannaCry hackers make a mistake and join the coins back together, those coins could become vulnerable to clustering and other blockchain analysis techniques," he said.
'Further, it is unclear how effective most mixers actually are, Heilman added.'
Source:
The World Is Watching: Can WannaCry's Creators Cash Out Their Bitcoin Ransom? - CoinDesk
