Unlike most of the people I know I've been expecting terrorist attacks against the U.S. We've been engaged in a whole host of abuses against the rest of the planet. Ranging for spraying pesticides over columbia, supporting dictatorships in third world countries to exploit cheap labor, supporting Israel in their fight with Palestine and the rest of the Middle East Area. The IMF and World Bank have forced extreme austerity upon a number of countries. Most of europe is peeved with the US for either backing out of or threatening to back out of the Kyoto treaty, the antiballistic missile treaty, and biological warfare treaty.
I expect that it was not a country or other large exposed organization. Whoever did this was capable of excellent planning, which suggests that they know that there would be retaliations, since the US could bomb most other countries to rubble, it's rather unlikely anyone would have been that foolish. But there's needless to say there's a long list of people who could attack the US, and really the only way to do such a thing would be via a terrorist attacks.