Europol stated to Panorama that £4bn’s worth of cryptocurrency has been laundered through Europe. Money laundering is one of several techniques that criminals use to obfuscate the movement of their assets so as to evade detection by authorities. Some have been known to utilise the Bitcoin blockchain for this purpose, attracted by its decentralised structure and notions of anonymity and pseudonymity. Here is the BBC article with a short analogy from the documentary that illustrates how a transaction on the Bitcoin blockchain works: http://www.bbc.com/news/technology-43025787. The full documentary is available on this link.
The Blockchain Intelligence Group works with law enforcement agencies and financial authorities to identify bitcoin-denominated funds that have passed through such illegal channels.