liberty island evacuated by boat around 12:30 pm friday , officials said .
bomb threat made by 911 who said ` they would blow up the statue '
police dog unit sniffed out potential explosive in locker on island .
but a full security sweep determined that it was a false alarm .
area was deemed safe at 4:30 pm , but island remained closed .
2,700 tourists were removed from the island ; had tickets refunded .