PARIS — French police hunted Tuesday for a second terrorist believed to have escaped after the bomb and gun massacres in Paris, while a U.S. official revealed that the suspected mastermind was part of an Islamic State cell that American intelligence agencies had been tracking for months. Meanwhile, France and Russia unleashed a new wave of airstrikes against IS targets in Syria, while fears of further terror attacks deepened in Paris and beyond. The Eiffel Tower closed to the public just a day after it had reopened and a soccer match between Germany and the Netherlands was canceled due to a bomb threat just 90 minutes before kickoff.