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How can police prevent shootings at large gatherings?

An investigation is underway after a gunman opened fire on crowds gathered for a Fourth of July parade in Highland Park, Illinois, that left at least six people dead and dozens of others injured. Former New York City Police Commissioner and NBC News law enforcement analyst, Dermot Shea, joins News NOW to explain how police can prevent shootings at large gatherings and when the investigation could go from local to federal. 
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