So most likely,
1) they didn't launder it properly, leading to police being able to trace it to their bank accounts. I wonder if tornado.cash was used.
2) then police had their names, leading to warrants for all online accounts - google account, apple account, etc.
3) they made the big blunder of keeping their private keys in their online account. Most likely a txt file in google drive. That is such a silly blunder. Without the private keys, the police has zero proof of anything. They could have made a hundred excuses for how they got money in their bank account, as long as the police didn't have the private keys. Who keeps their private keys in an online account?
Apparently the biggest criminals make too many silly mistakes. The old saying applies here: "you don't have to be smart, just don't be an idiot"
Also, with mixers such as tornado_cash, laundering money is ,sadly, pretty trivial.
The difference is that laundering provides you with an explanation for wealth and/or income. Example of laundering: buy a business (with clean or borrowed money), have fictional customers "spend" their cash money at your business every day, then report your income and pay taxes. Now if anybody asks about where you got your money, you have a seemingly legit explanation.
Mixing does none of that. So mixing may be trivial, but laundering is not.
edit: now that I think about it, is that why NFTs are so popular? Are people pretending to have gotten capital gains, while in reality they're buying these things from themselves? That would explain a lot.
Then you can convert those crypto (in new account) into fiat money.
Everyone will know you are lying, but they will never be able to prove it.