[Aaronstotle]

Bitcoin is a public blockchain, there are various blockchain analytic firms such as Elliptic/Chainalysis that offer bitcoin tracing services.

Bitcoin is not private nor anonymous, the rise of blockchain surveillance is why privacy coins like Monero are gaining in popularity.

That being said, I'm sure it wasn't solely BTC transactions, these guys seemed to have very poor op-sec for performing such a big hack.