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"
As far as how exactly they got caught, there was a reward offered by the company it was stolen from. It may have been someone tipped the feds off for the reward.
Why anyone with a significant amount of crypto assets isn't going to insane extremes in terms of secrecy and durability is beyond me.
It would seem to solve a lot of just organizational problems where "jan is out of the office today" and nobody can do the thing ... but if access is spread out among 10 people ... 3 probably are in the office when needed.
Granted I've never seen it used in production personally, not / seen it on a granular level.
We would first go through the process with 'dummy' keys to check everyone was happy with the process and what we were going to do (ie. which commands, what software, what exactly will be signed). We would then do it again with the real thing. And then we'd power off the computer till next time it needed to be used.
"Clunky" would be a good way to describe it... But it's hard to make it better without relying on a bunch of software we don't have the resources to audit.