Marathon madness: Terrorist attack shocks America

A bomb explodes Monday near the finish line of the Boston Marathon. With at least three dead and more than 100 injured, the bombing of Boston's cherished running event is the worst terrorist attack on U.S. soil since Sept. 11, 2001. /WBZ-TV image

Monday's bomb attack on the Boston Marathon is a reminder that the United States faces an insidious struggle with domestic and foreign terrorists. Whoever is responsible for the marathon bombings can expect retribution akin to the U.S.-led invasion of Afghanistan that followed the 9/11 terrorist attacks on New York City and the Pentagon.