Before Boston: Marathons marred by violence

Two mourners hug during a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013.</p>

Two mourners hug during a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013.

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.