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British Prime Minister Theresa May increased the terror alert level from severe to critical in the wake of Monday night's terrorist attack in Manchester, England, which killed 22 people and injured dozens more who were leaving a leaving a concert performance by Ariana Grande.

Grande, an actress and pop and R&B performer, consistently packs arenas around the globe with audiences that are largely made up of teens and young adults. The youngest confirmed victim of this week's attack was just 8 years old.

It is not the first time terrorists have attacked venues where young people have celebrated. The Pulse nightclub in Orlando, Florida, and the Bataclan in Paris, France were filled with young adults dancing and listening to music. While the actual motivation behind this week's attack is still unknown, it awakens yet again the feeling of vulnerability in places that are supposed to be safe.

Jessica Stern, a professor at the Pardee School of Global Studies at Boston University and co-author of "ISIS: The State of Terror," explains how terror groups evoke fear by targeting the most vulnerable parts of the population. 

This segment is hosted by Todd Zwillich.