In theory mixing services are used to disguise the origin of the coins.
In practice - not sure if this works. The FBI has been using software to track coin movements. There are lots of start-ups that track bitcoins, and some might be able to track mixing services too:
https://bravenewcoin.com/news/bitcoin-tracking-platforms-gaining-traction/it is a large task to track and trace the movement of every bitcoin. A blockchain startup, called SABR, claimed that its software could have completed the task in a matter of days.
SABR describes its product as a “Palantir” for the blockchain, a name used by revered author J. R. R. Tolkien. The author gave the name to crystal balls used for communication, and as a means of seeing events around of the world.
SABR’s mission is to provide law enforcement with a view beneath the surface of multiple blockchains, to ensure digital currencies are not being used for illicit purposes, and to bring legitimacy to digital currencies. The surveillance team explained that blockchain technology, in conjunction with a distributed and aligned transaction confirmation network, has the potential to be the most useful innovation of the 3rd millennium.
The New york based startup raised US$1m in seed funding for its patented technology, and said it will use the investment to develop its blockchain monitoring and identification system.
Another startup which intends to identify illicit activity, United Kingdom based Elliptic, is also tackling the situation. Having already raised US$2m, from Octopus Investments during 2014, the company recently announced an additional US$5m had been secured for development.
Elliptic’s bitcoin transaction identification software implements an artificial intelligence-based technology, which can explore the Bitcoin blockchain. Elliptic will use the technology to expose patterns in the network which may be suspicious transactions or activities.
The European Cybercrime Centre (EC3) has also taken an interest in taming the Wild West of bitcoin transactions. EC3 has partnered with blockchain company Chainalysis, an official investigator for the creditors of the collapsed bitcoin exchange, Mt Gox. Chainalysis will provide the law enforcement agency with access to software which detects suspicious activity in real-time, alongside other investigation tools.