The US probably runs a huge number of TOR nodes, and can therefore do correlation analysis* to unmask TOR users. I'm not an expert, so don't take this statement at face value, but that's my understanding. Also keep in mind that if your threat model includes the NSA or similar agencies, you are probably overly paranoid or else you are fucked.
*edit to add: this is called a "Sybil" attack. Look it up in relation to TOR. Again, this doesn't mean that TOR isn't perfectly secure for most threat models, but neither is it impervious.