Well-wishers pay their respects at a makeshift memorial at the national headquarters of Turning Point USA in Phoenix after the shooting death of Charlie Kirk, CEO of the organization, on September 10. Ross D. Franklin/AP

As gunshots pierced the air from high above, people in two different American cities this month found themselves running from a deadly shooter they could not see.

The nearly undetectable threat of snipers has security professionals sounding the alarm as age-old tactics of assassination become the tool of choice for modern killers often fueled by ubiquitous streams of online hate.

Snipers have been a potential threat for centuries, but security experts say the current toxic United States political climate coupled with the threat of rapid online radicalization and e

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