I think everyone has known this happens and has happened for years. There have always been market makers and crypto is more like a game of hot potato and than anything else.
Bloomberg's sources said that the investigation will focus on practices that have essentially been regulated out of futures and equities markets. Traders submit false orders and cancel at the last moment (spoofing) or trade heavily with themselves to create a fake impression of demand (wash trading). Both practices are illegal. Bloomberg also notes that virtual currencies can be ripe for fraud as evidenced by wild price swings, a fragmented trading presence and a lack of regulations (until now). The Justice Department is working with cryptocurrency regulator, the Commodity Futures Trading Commission, in its investigation.