Her's some actual legal information about the seizing of BTC by the FBI:
In June 2014, the United Nations Office on Drugs and Crime (‘UNODC’) published a ‘Basic Manual on the Detection and Investigation of the Laundering of Crime Proceeds Using Virtual Currencies’ (hereinafter ‘Manual’) providing a guide as to how the Bitcoin can be located and seized.
The advantage of locating and seizing the Bitcoin wallet is that the potential complex jurisdictional issues that the use of virtual currencies creates, (as it operates in the online environment that blurs the national borders) can be avoided. The physical location of the instrument containing the Bitcoin wallet will in most cases be considered as the rightful jurisdiction for the purposes of freezing, seizure and confiscation.
Despite the technological difficulty in the early detection of illegitimate use of Bitcoins, it is nevertheless possible although difficult for Bitcoins to be seized as proceeds of crime. The most effective way law enforcement authorities can seize Bitcoins is by obtaining the private keys which are linked to the Bitcoins or physically confiscating the ‘Bitcoin wallet’.
Essentially once they have court approval and know where the wallet is - hard drive, USB drive, digital wallet. They will confiscate the wallet (similar to physical property seizures from a drug dealer) then just transfer the BTC to an account or wallet they control.
The above article is from Australia, but the manual is used internationally so I'm assuming it would be the same for the US:
https://www.lexology.com/library/detail.aspx?g=40d618a0-89e0...