In this case, our fraud team has needed more time than usual as the case is quite complicated
The proper thing to do is to, in the worst case scenario, void the bet and return the deposit. You should not arbitrarily seize goods that aren't yours. I wouldn't do it if I was running a casino, anyway.
Return of deposit should be done without any questions being asked, immediately.
The return of winnings - if they wish to prolong it - can be "subject to investigation" (say because they investigate the bets, not the account) ... but they haven't chosen this path anyway, they have chosen to "investigate" his account - which is cheating, they are the ones cheating and lying to their customers.