If they wanted to they can, it's easy as they write a letter to the exchange where our coins are with a gov order to freeze that money, shouldn't be a big deal for them to do it. But if the bitcoins are under our wallet then it would be a lil harder for them because they have to get access someway to our machine to access to the private keys of the coins.

The FBI has some advanced tools and zero days to access any device. So, if they want to get your coins they will be able to do it.