While Hale was undercover, New York City (then the area at the southern tip of Manhattan, mostly south of what is now Chambers Street) fell to British forces on September 15, and Washington was forced to retreat to the island's north in Harlem Heights (what is now Morningside Heights). Shortly after, on September 21, a quarter of the lower portion of Manhattan burned in the Great New York Fire of 1776. The fire was later widely thought to have been started by American saboteurs in order to keep the city from falling into British hands, and though setting fire to New York during Washington's retreat had indeed been proposed, Washington and the Congress had rejected the idea and denied responsibility. The Americans accused British soldiers of starting the fires without orders from their superiors so they could sack the city. In the fire's aftermath, more than 200 American Patriots were detained by the British for questioning.