Ambulances and police are seen at the blast site after an explosion in Istanbul's central Sultanahmet district. At least 10 people were killed and 15 wounded in a suicide bombing near tourists in central Istanbul's historic Sultanahmet district. Can Erok/Getty Images hide caption

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Members of the Los Angeles Police Department's elite Metropolitan Division participate in a simulation of a Paris-style coordinated attack. The team was tested on response time to converge on this site. Martin Kaste/NPR hide caption

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After recent terrorist attacks, social media companies are under pressure to do more to stop messaging from terrorist groups. Patrick George/Ikon Images/Getty Images hide caption

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Olympic team members stand on the field as the Olympic flag flies at half-staff during a memorial service in the Munich Olympic Stadium in Germany on Sept. 6, 1972. Eleven Israeli team members were killed by Arab guerrillas at the Summer Olympic Games. A crowd of 80,000 filled the stadium to capacity. AP hide caption

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President Obama, photographed from the Rose Garden, delivers a prime-time televised address marking the the end of combat mission in Iraq from the Oval Office in 2010. Susan Walsh/AP hide caption

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A soldier patrols outside a Brussels shopping center Saturday, after the government put the city on its highest terror alert. All metro train stations in the city are now closed. John Thys/AFP/Getty Images hide caption

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A woman lights a candle at the Place de la Republique in Paris on Friday, marking a week since the terrorist attacks. Pascal Le Segretain/Getty Images hide caption

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People gather to look at a burnt car following a bomb explosion that rocked a roundabout near the crowded Monday Market in Maiduguri, Nigeria, on July 1, 2014. The city has been repeatedly hit by Boko Haram terrorists. A recent report found that in 2014, the group was responsible for more known deaths from terrorism than any other organization. Stringer/AFP/Getty Images hide caption

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Parents walk their children to school on Monday near one of the venues attacked in Paris. Kyodo /Landov hide caption

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People gather outside Le Carillon, a bistro in Paris' 10th arrondissement that was attacked on Friday. Jerome Delay/AP hide caption

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Parisians gather around at the Place de la Republique in the aftermath of Friday's terror attacks. In the 1980s, writer Joji Sakurai lived nearby in a vibrant, multicultural neighborhood. Uwe Anspach/DPA/Landov hide caption

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