A Yemeni soldier searches a car near the airport in the capital, Sanaa. The United States has ordered Americans to leave Yemen immediately amid a warning of a possible attack. Mohammed Huwais/AFP/Getty Images hide caption

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A Yemeni soldier stands guard Tuesday near Sanaa International Airport. Mohammed Huwais /AFP/Getty Images hide caption

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An army trooper sits beside a machine gun that is mounted on a patrol vehicle at a checkpoint in Sanaa, Yemen. Security is tight in the capital amid warnings about possible terrorist attacks. Khaled Abdullah /Reuters /Landov hide caption

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A checkpoint leading to the closed U.S. Embassy compound in the Yemeni capital, Sanaa, on Saturday. Mohammed Huwais /AFP/Getty Images hide caption

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A Nov. 23, 2010, photo shows Michael Adebolajo (center, in dark T-shirt) with suspected Al-Shabab recruits who were arrested by Kenyan police. Adebolajo, one of the main suspects in the brutal murder of a soldier in London, was discharged from the hospital Tuesday. AFP/AFP/Getty Images hide caption

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As Ian Rigby spoke Friday about his stepson Lee, a British soldier who was murdered on a south London street this week, the young man's widow, Rebecca (right), and his mother, Lyn, reached out. Dave Thompson /Reuters /Landov hide caption

itoggle caption Dave Thompson /Reuters /Landov

U.S. Army Maj. Nidal Hasan is charged with a 2009 killing spree that killed 13 people at Fort Hood in Texas. Homegrown terrorism by Muslim Americans has been growing over the past decade. HO/AFP/Getty Images hide caption

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