Four bombs went off in 26 minutes this morning in three London subways and a double-decker bus. At least 37 people were killed and 700 injured. As claims of responsibility are being investigated, Prime Minister Tony Blair branded the attacks "terrorism," timed to coincide with the G8 summit in Scotland. Calling the explosions the work of Islamic extremists, he added that most Muslims deplore the attacks and warned that "the slaughter of innocent people must not succeed." Officials in London say that long experience has made them prepared for terrorism. US Secretary of Homeland Security Michael Chertoff says there's "no credible information" about plans for attack in this country, but he's raised the threat level--on transit systems only--from Yellow to Orange. We get updates from London and look at the vulnerability of American cities to urban terrorism with journalists, eyewitnesses, and experts in terrorism, defense and security, including a former special assistant to President Bush and the ranking member of the US House Committee on Homeland Security.