Take all their hardware, storage media and notes (including online data storage services); interrogate the person; find out what accounts (and 3rd party sites) were involved from various sources including that interrogation, analysis of stored data, and internet wiretaps/logs to obtain the neccessary credentials, and transfer those bitcoins to a separate wallet that the feds control.

If some bitcoins are stored at some 3rd party providers such as Mt Gox, subpoena them to freeze all such assets and transfer them to feds.