According to CNBC, federal prosecutors had launched a criminal probe into the cryptocurrencies earlier this year based on suspicions that their price was being manipulated.
bitcoin have many years since its creation and its price has gone up a lot and fallen a lot, I do not know because only now they started to want to investigate about a possible manipulation of prices and many exchanges are not in the USA and do not have license since that still an unregulated market, this is all very difficult.
I think it might be the main reason behind the price of bitcoin dropping to around $4000
I do not believe that is the reason for the fall in prices