A senior law enforcement official refused tonight to say how many active cells the authorities believe may still be operating inside the United States.

Scientists know a ton about how different cells operate.

But he did report that slightly more neutrons were detected while the cell was operating than could be expected from normal sources.

The scattered cells of the terrorist network still operate in many nations, and we know from daily intelligence that they continue to plot against free people.

Finally, many cells in the kidney normally operate under hypoxic conditions.

The cell was operating in an area 12 kilometers southwest of the Iraqi capital.

A typical covert cell operates as anything from a lone individual to a small group.

A women's cell and a narcotics control cell also operate in the city.

The vehicle was unusually well insulated, and the solar cells also operated a small heater.

The cells operate based on the photoelectric effect.