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The Boston Marathon was not the first race to become the scene of violence and fear. Here's a timeline of past incidents.
In the last 20 years, at least eight marathons have been threatened by violent attacks, according to the University of Maryland's Global Terrorism Database. Some attacks were thwarted, as was the case at the Belfast Marathon three separate years. Other races ended in bloodshed, as in Sri Lanka in 2008, when 14 people were killed by a suicide bomber.