Here is a link explaining the US civil forfeiture law which can be used to seize assets:
https://www.justice.gov/afp/types-federal-forfeiture
Specifically, I'm talking about in rem forfeiture here:
In rem (against the property) court proceeding brought against property that was derived from or used to commit an offense, rather than against a person who committed an offense. Unlike criminal forfeiture, there is no criminal conviction required, although the government is still required to prove in court by a preponderance of evidence that the property was linked to criminal activity.
Theoretically, bitcoin that has been mixed could be deemed proceeds of a crime after yesterday's indictment. I don't know whether the government would pursue this course of action.
There are many instances where this in rem civil forfeiture has been used by governments to take property in what seems an unjust manner.
Here's the classic example:
A husband borrows his wife's car and solicits a prostitute. Prostitution is illegal, and the car is seized as property used in the commission of a crime. The car is seized, though the innocent wife owns it. The theory is that the action is brought against the car, not the wife.