We don't have any conclusive evidence yet, but let's be perfectly frank: the bomb was an IED, and the person, persons or organization who planted it are home-grown terrorists, part of the network of militant anti-choice terrorists across the United States. There have been active anti-choice terrorist cells in the US since Roe vs. Wade. When the War on Terror was first declared by this administration, it was these people that first came to my mind. I knew full well that Bush would never allow such organizations to be publicly investigated; they're his base, after all.
Your war on terror begins at home, Mr. Bush. The "We're fighting them over there so we don't have to fight them here" rhetoric doesn't work when they're already here. If your focus for this larger war is terrorism, deal with the anti-choice terrorists here in the States. If your focus is fundamentalist extremism, deal with the fundamentalists of your own religion.
Of course, it seems like your focus is neither of these things, but instead on keeping the American people in a nonstop delirium of fear.