Cell phone signals are being used by law enforcement officials to find missing people in romote areas, to track terrorists and fugitives, and to place suspects near crime scenes, experts say. "The average citizen is not aware that they are carrying a location-tracking device in their pocket," said Kevin Bankston, a lawyer for the Electronic Frontier Foundation, a San Francisco-based group that works to preserve privacy rights.

When turned on, cell phones constantly emit locator signals called pings so their companies know to which towers to route phone calls, Bankston said.

Investigators can obtain logs from wireless companies containing such data to track people's movements, he said. In urban settings with many towers, the location can be narrowed down greatly - to within blocks. In more rural settings with fewer towers, a more general location can be established.

Most new phones also contain Global Positioning System chips that communicate with satellites, allowing authorities to pinpoint a precise location of the handset. The chips are one way companies can comply with federal rules designed to give emergency dispatchers more information on the location of cell phone callers.

Just how often law enforcement has used the technology to track down a body or help solve a crime was not available, mostly because agencies are reluctant to discuss their investigative tools.

Bill Hagmaier, executive director of the International Homicide Investigators Association, said almost all major police agencies employ the technique to crack cases.

"It's an outstanding tool. Who doesn't carry a cell phone these days? Cell phones are almost as popular as jewelry and wallets," he said. "It's a fairly new investigative tool but it's one that is certainly growing in use."