It is not within their power to seize these assets. And that is the reason why the judge (Steve Kim) rejected the request from the FBI to keep the valuables with themselves. The judge was very clear when he said that if the FBI want to seize these assets, then they need to present solid evidence to prove that they are proceedings from crime (FBI was arguing for the reverse scenario - the owners should first present evidence to claim that these assets are not linked to any criminal transaction). So we can say that at least in this instance, the FBI is not getting what they wanted.
FBI has always been the most horrible place in all of USA and they have done so many crimes under the laws protection that they literally killed people they wanted to kill without facing any consequences, hell there were citizens rooting for it.
Go check out cointelpro and you will see how dirty their hands are, and look at all the Latin America works as well and you will be disgusted. In any regular nation a place like FBI would be closed down very quickly, they are not "investigating" anything like their name suggests, they are taking action and those actions are just to make themselves more and more powerful that's it, so they are very dangerous.
I am glad that judge decided to not give them the belongings because it was obvious that they were just doing it to make more money, it wasn't about any crime or not, it literally had nothing to do with that, they were all like "hey we raided a place and found money, we gonna keep it!!" and judge ordered it to be given back, if not then FBI would keep it illegally.