President Obama at news conference
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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
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