Video taken by a bystander and posted on Twitter showed people running through the lobby for the exits.
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Police Department spokesman Stephen Davis said the backpack was found near the entrance to a Nike Town store in the building.
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The bomb squad checked out the package and gave an "all clear" just after 5 p.m.
President-elect Donald Trump lives in the tower and has his offices there, though he is presently at his Mar-a-Lago estate in Florida.