The FBI can seize Bitcoin if the private keys are insecure by way of being physically available and unencrypted on servers/devices domiciled in the United States, or by cooperation of a party with access to said private key.
The FBI, or any government entity, cannot seize Bitcoin from an address otherwise. To do so would demonstrate the ability to break the encryption underpinning Bitcoin's opensource key creation protocol.
Put another way: there's a billion dollars sitting at addresses right now for you to steal. If you can crack the keys, go steal that money and make it public. That would destroy Bitcoin quickly. The open market bet is that you (or anyone else) can't.
The FBI, or any government entity, cannot seize Bitcoin from an address otherwise. To do so would demonstrate the ability to break the encryption underpinning Bitcoin's opensource key creation protocol.
Put another way: there's a billion dollars sitting at addresses right now for you to steal. If you can crack the keys, go steal that money and make it public. That would destroy Bitcoin quickly. The open market bet is that you (or anyone else) can't.