How terror attacks changed the US
President Obama has said it still is not clear whether the attack on Boston was the work of a foreign or domestic group, or what he called a "malevolent individual".
Since the 9/11 attacks in 2001, the US government has invested billions of dollars in schemes to protect Americans from acts of terrorism at home.
The BBC's North America editor Mark Mardell reports.
16 Apr 2013
- From the section UK