Why are the US and UK probing a $20B cryptocurrency transfer?

Could you elaborate on the reasons behind the United States and the United Kingdom's decision to investigate a $20 billion cryptocurrency transfer? This seemingly massive transaction has raised significant concerns in the financial and regulatory world, and I'm curious to understand the underlying motivations for this inquiry. Are there specific suspicions of illegal activity or money laundering involved? Is there a perceived threat to the stability of the financial system? Or are the authorities merely seeking to better understand and regulate the emerging cryptocurrency market? I would appreciate a detailed explanation of the factors driving this investigation.