NEW YORK -- The FBI was scrambling.
Agents had intercepted information about a possible terrorist attack in Manhattan, including a diagram showing a mysterious device. The raw intelligence was relayed to experts in Washington, who offered a daunting diagnosis: "You have a problem."
As chilling as that sounded, the situation wasn't real. But authorities say it could be, and what followed over the next two days was an ambitious stress test of the city's line of defense against a radiological or nuclear terrorist attack.
The exercise earlier this week involved hundreds of New York Police Department officers and FBI agents trained at detecting threats, along with an elite unit of federal weapons experts expected - with the approval of the U.S. attorney general - to swoop in by plane and defuse them.
There have been no specific threats against New York City. But since the Sept. 11, 2001, terrorist attacks, law enforcement officials have repeatedly warned that the city remains atop terrorists' hit lists - and that a radiological or nuclear device could be in their arsenal.
"It's something we're very concerned about," said Joseph Demarest, head of the FBI's New York office.
Authorities say a small nuclear bomb could cause widespread devastation. Failing to intercept it before it's detonated is not an option.
"Hopefully, we'll never have to do this," Don Alway, an FBI counterterrorism supervisor, said of the deployment. "But if we do, we have to do it right the first time."
Much of the drill played out behind the scenes and only select specifics were made public.
But interviews afterward with Alway and Richard Falkenrath, a top NYPD counterterrorism official, offered a glimpse of how federal and local authorities would respond in tandem if the threat ever become reality.
Investigators with the Joint Terrorism Task Force were first offered "purposely vague" information to test their skills at assessing potential threats, Alway said. Conference calls with officials and experts from the FBI, and departments of Defense, Energy and Homeland Security in Washington about the suspicious diagram confirmed it showed an improvised nuclear weapon.