I think technical attacks would be much more affordable.
If you can stop BTC from working or hinder it's operations the value of it is lowered. So just invest in hardware and use it in harmful ways...
There is really quite a lot of options and multiple attack routes if you start thinking about it. Why not just write more of those nice viruses to destroy wallets, ddos the online services and maybe even nodes in network...
I was thinking more along the lines of a "legal" way to bring down the network. Virsus's and DDOS aren't exactly legal.
Umm, since when has USA government cared about what is legal? It's war on terrorism, anything goes...
If they do it, it's legal for them... Or at least the consequences are very minor...