NEW YORK (AP) – 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.

The exact whereabouts of that weapon? Unknown. But the expert response team quickly assembled Tuesday and boarded a plane at an unspecified location and flew to John F. Kennedy International Airport. They were carrying “all the technology needed to defeat a weapon of mass destruction,” Alway said.

The NYPD was put on alert as more intelligence came in suggesting that the device may be on a foreign ship entering New York Harbor. The nation’s largest police department deployed a radiological dragnet.

Under a federally funded program called Securing the Cities, the NYPD has equipped officers with hundreds of radiation detectors. Some are the size of a cell phone and carried by individual officers. Others are larger, more akin to radar or speed guns and sometimes mounted on cars, harbor patrol boats and helicopters.

Under rain clouds Tuesday evening, patrols began screening the endless flow of traffic entering Manhattan through bridges and tunnels, Falkenrath said. In a separate test, the NYPD planted safe amounts of radioactive material – simulating less-powerful dirty bombs – in five unmarked test cars. All were successfully intercepted on the Brooklyn Bridge and several other bridges.

The hunt for the nuclear device, meanwhile, was narrowed to a busy Queens expressway. Still unsure of what they were looking for, highway units narrowed traffic to one lane and used two unmarked vans equipped with more sophisticated devices that can detect nuclear threats to take readings on each vehicle passing by.

Approaching midnight, police got a hit on a red SUV that was quickly stopped. Supervisors at the scene took note on whether officers took proper precautions approaching the vehicle.

In back was a fake device specially designed by the Department of Energy to emit safe levels of neutrons, along with two technicians charged with protecting it.

The discovery triggered a series of notifications – to the NYPD bomb squad, top New York City law enforcement officials and higher-ups in Washington. The team expected to defuse the device – after it was picked up by FBI vehicles and escorted by police through the city – went through a separate exercise the next day to assess and disable the mock bomb.

Police Commissioner Raymond Kelly called the exercise a success.

“We plan for the worst with the best exercises possible,” Kelly said Friday. “This one was realistic – at night, in the rain, with traffic. We put our personnel to the test and they performed well.”

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