Of course they can. In the case of Silk Road they immediately swept the coins into their own Bitcoin address on October 2 (https://blockchain.info/address/1F1tA

Of course they can. In the case of Silk Road they immediately swept the coins into their own Bitcoin address on October 2 (https://blockchain.info/address/1F1tAaz5x1HUXrCNLbtMDqcw6o5GNn4xqX?sort=1 https://blockchain.info/address/1F1tAaz5x1HUXrCNLbtMDqcw6o5G...), the same day they seized and shutdown Silk Road (http://news.yahoo.com/fbi-raids-alleged-online-drug-market-silk-road-153729457.html http://news.yahoo.com/fbi-raids-alleged-online-drug-market-s...). No one with the Silk Road keys can take them back, it doesn't matter that the FBI is still holding the Bitcoins. They'll only get better at this.