Within the last two months a U.S. government sensor detected an unauthorized shipment of radioactive material crossing from the Canadian border into the Detroit area, Fox News has learned. 

Radioactive material is the key ingredient in a so-called "dirty bomb," a conventional explosive that when detonated contaminates an area with radioactivity.

Bush administration officials told Fox News that as of now they have no conclusive intelligence information to indicate that a terrorist dirty bomb has been built.

But administration officials did confirm that elite mobile Nuclear Emergency Support Teams (NEST) have been dispatched throughout the Great Lakes region to search for the material, and that search continues.

Officials said it is impossible to tell exactly what the radioactive material was. It could be an unregistered shipment of heavy-duty construction or medical equipment that uses radioactive material; there is even the possibility that the detection devices raised a false alarm.

Since the Sept. 11 attacks NEST teams have been called out as many as 70 times to investigate "communicated nuclear threats or illicit trafficking of nuclear materials," sources told Fox News. 

Officials say a NEST team's normal response time to a location is about four hours. In this particular case the location was described as a major point of entry to the U.S. from Canada in or around Detroit.

In all of those other cases the government has run down the leads and found no threat in a matter of just a few days — in this case it has been two months and they have yet to close the investigation.

NEST teams are run out of Department of Energy labs and NEST overall has a $90 million budget. The teams are transported to incident sites by the military.

At any given moment there are about 70 full-time individual first responders waiting in teams of five to 10 individuals. There are another 900 or so individuals on call. The teams are staged strategically all over the country.

NEST teams were created in 1975, for years their mission was viewed largely as accident response — now it is terrorism.

NEST teams use both mobile and stationary detection equipment.

Helicopters outfitted with detectors can sweep from above as "hotspot mobile labs" in vans patrol streets and passive monitors strategically placed in bridges, tunnels and other locations send an alert through a network when they are tripped.

Some passive monitors "store and forward" information. These monitors do not transmit immediately, so data is collected and then sent at intervals, making the alerts less timely.

Individuals can be equipped with briefcase-sized detectors and even handheld devices.

The detectors register gamma and neutron radiation.

NEST sweeps have been conducted periodically in Washington, New York, various ports, and elsewhere since October. The FBI randomly selects cities each month for NEST sweeps.

Carl Cameron currently serves as Fox News Channel's (FNC) Washington-based chief political correspondent. He joined FNC in 1996 as a correspondent.