If the money to purchase bitcoins was acquired in an illegal manner can authorities lay claims to the money.
If Bitcoin is regulated in a certain country and If the government will found out that the Bitcoin is came from illegal source then of course they can hold and seize the account but the problem is it will be difficult for the government to determine if Bitcoin is illegal so therefore they will just mostly rely on Bitcoin to fiat conversion in the local exchange then they will start monitoring and investigating on how a person posses such a huge money from Bitcoin.