Why the Boston Marathon became a target

Two explosions rocked the Boston Marathon as runners crossed the finish line on Monday, killing at least three people and injuring over 130 on a day when tens of thousands of people packed the streets to watch one of the world's best known marathons.

News agency Reuters compiled a list of facts about the Boston Marathon:

 The race is always held on Patriots' Day, a Massachusetts state holiday held on the third Monday in April, which commemorates the earliest battles of the American war for independence.

The race was first held in 1897 with 18 participants, making it the world's oldest annual marathon.

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The 2013 Boston Marathon, which drew about 27,000 runners, honoured the 26 people who died at the Newtown, Connecticut, school shooting with a special marker at the end of mile 26.

The race runs through eight Massachusetts cities and towns: Hopkinton, Ashland, Framingham, Natick, Wellesley, Newton, Brookline, and Boston.

It begins in Hopkinton and ends at Copley Square, alongside the Boston Public Library.

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The Boston Marathon is considered one of the more difficult marathon courses because of the Newton hills, which culminate in Heartbreak Hill near Boston College.

Boston Globe reporter Jerry Nason coined the name Heartbreak Hill after John Kelley overtook competitor Tarzan Brown on the Newton hills in the 1936 race, tapping him on the back but ultimately losing his lead to Brown on that final hill.

Bobbi Gibb was the first woman to unofficially complete the race in 1966. Women were not officially allowed to participate until 1972.

Ethiopian runner Lelisa Desisa won the men's division in the 2013 Boston Marathon, finishing in 2 hours, 10 minutes and 22 seconds.

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Kenya's Geoffrey Mutai holds the course record with 2 hours, 3 minutes and 2 seconds, which he ran in 2011.

The youngest runner to win was Tim Ford, who was 18 when he won in 1906.

Clarence Harrison DeMar is the only marathoner who has won seven times at Boston. His seventh victory in Boston came in 1930, when he became the oldest winner of the event at the age of 41.

Prize money was awarded for the first time in 1986 and the top finishers now compete for more than $800,000.

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The Boston Marathon is the only marathon in the United States that maintains qualifying times and specific requirements that generally state a runner must have completed a qualifying marathon within the year and a half before Boston.

In 1980 Rosie Ruiz created a scandal when she cheated to win by taking the subway part of the way.