they never hacked into his computers, and whether or not they can follow them on the blockchain is irrelevant.
Perhaps I am mis-remembering about the hacking. But, from day 0, the police can buy stuff on the site, pay in bitcoins, and follow them on the blockchain. That tracing could reveal if there were other wallets beyond those that that they seized (~30'000 on the server and ~150'000 on his laptop).
It's a good thing for law enforcement that no one ever invented a tumbler, or any other method to hide bitcoins eh
If the cops had any brains they would set up a dozen fake tumbling services, with unbeatable fees and spiffy interfaces; and quietly close or co-opt the legitimate ones. But fortunately they are nowhere as smart as the typical users of such services.
i doubt that would be legal, atleast in germany im pretty sure that the the police are not be able to do stuff like this by law.
(i mean from buying on SR or etc. to setting up tumbling services)