It would be difficult to be sure that there is only one person with the key. If the defendant has a co-collaborator who also has access to the keys and is outsi

It would be difficult to be sure that there is only one person with the key. If the defendant has a co-collaborator who also has access to the keys and is outside the reach of the seizing authority (or unknown to them), they could just transfer the Bitcoins to another new address and out of seizure. The seizing authority in general has no way to tell in advance if such a co-collaborator exists, and assuming the transfer is to a new address with no historical transactions, it would be extremely difficult to identify who the collaborator was after the fact.