How did the FBI "seize 260000 BTC"? http://www.zerohedge.com/news/2013-10-08/bitcoin-1-0-fbi

How did the FBI seize 26000 BTC? I thought that all wallets are supposed to be safe. Can you give the technical details?
To participate in Silk Road, users had to deposit funds with Silk Road. The FBI gained access to the private keys corresponding to these user balances. Thus, the FBI was able to transmit the funds to an FBI controlled wallet and confiscate the assets.
Following up on the question how they might have gained access:
The software running on the Silk Road server must have had spending access to the Silk Road users' internal balances. Therefore, there must have been a mechanism to unlock the wallet (assuming an encrypted wallet was used). The passphrase would have been available to the program, concluding from the outcome, likely in the files on the server. As the server was probably running when it was seized, the FBI might have been able to access the files directly and thus gain access.