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Gary LaFree

FILE - In this Feb. 3, 2017 file photo, police officers cordon off the area outside the Louvre museum as a soldier opened fire after he was attacked in Paris. With the scars and threat of extremist violence looming over France's presidential election, voters will cast ballots under a state of emergency that is becoming a part of the fabric of French life. (AP Photo/Christophe Ena, File) (AP)

6 reasons why stopping worldwide terrorism is so challenging

Gary LaFree
FILE - In this March 22, 2016 file photo, Metro-North Railroad police officers with a police dog patrol Grand Central Terminal, in New York. The best line of defense against a Brussels-style attack in the nation's largest subway system may be man's best friend. The NYPD this week graduated eight new dogs specifically for its elite counterterrorism unit. (AP Photo/Richard Drew)

6 reasons why stopping terrorism is so challenging

Gary LaFree