After the arrest of william ulbricht, the operator of Silk Road.
the website which was used for online selling of drugs, and which used bitcoins for funding.
and that is because of the reason of privacy and protection policy of bitcoin wallet.
the only way to move bitcoin out of someones wallet needs the private key to authorize the transiction.
and FBI has been unable to overcome all the encryption used in the wallet.
source: www.techworm.net
