Speaking of the two bombs that killed three and wounded dozens at yesterday's Boston Marathon, President Obama today announced, "Given what we now know about what took place, the FBI is investigating it as an act of terrorism." The crime scene is 12 blocks long in the heart of a city still in a state of shock. There have been no arrests and no one has claimed responsibility. Praising "exhausted runners who kept running to the nearest hospital to give blood, and those who stayed to tend to the wounded," and medical staffs and students at several local hospitals, he called on all Americans to be on the alert. We talk to Bostonians and update the investigation. What can be learned from the history of terrorism in the United States?