At the Federal Bureau of Investigation’s counterterrorism compound in the Chelsea neighborhood, the urgent announcement sends special agents rushing to the streets.

Has a secret terrorist cell been uncovered? A trove of bomb-making materials, perhaps?

The warning, it turns out, often concerns a matter far more mundane — even as it is universally alarming:

“If you’re parked on the south side of 18th Street, they’re towing cars.”

For decades, the F.B.I.’s vehicles were off limits to New York City tow trucks, given the bureau’s authority. But over the last few years, the Police Department has towed hundreds of cars belonging to this and other governmental agencies. Not even police vehicles are exempt.

The cause of all this is a small towing unit of the department’s Internal Affairs Bureau, charged with enforcing certain parking violations against government vehicles that might otherwise enjoy some latitude.