A device containing radioactive materials disappeared from a construction site in central Fairfax County last week, prompting federal authorities to warn yesterday that whoever has the device could be exposed to high levels of radiation.

Spokesmen for the Nuclear Regulatory Commission and county police said they believe the device, known as a Troxler soil moisture density gauge, was stolen Friday night or early Saturday, probably by someone who did not know the device is hazardous.

Troxler gauges, commonly used around construction sites to measure the solidness of the ground, emit 25 rems of radiation per hour when turned on, an amount more than eight times the annual exposure limit for nuclear power plant workers, according to NRC spokesman Kenneth Clark.

However, the highest levels of radiation would occur only within a few inches of the gauge, according to Clark and the device's manufacturer, North Carolina-based Troxler Electronic Laboratories Inc.

A person three feet or so from an operating gauge would suffer serious health effects only if exposed to the radiation for prolonged periods, said Ken Brown, a Troxler officer.

The device -- which resembles a large beige telephone, about 15 inches long and nine inches wide -- was reported missing to the NRC on Tuesday, after an employe of Alexandria-based ATEC Associates of Virginia Inc. left it unattended outside her car at the Oakton construction site Friday night, an apparent violation of NRC regulations, agency officials said yesterday.

Officials with ATEC Associates declined to comment.

Fairfax County police Officer Bill Coulter said that ATEC officials told him the unidentified employe who left the device had been fired.

The employe was locked out of her car about 5 p.m. Friday at Oakton Gable, a condominum development under construction on Arrowhead Road, a short distance from I-66 and Rte. 123, Fairfax and NRC officials said. Locked in the car, they said, was a carrying case and a padlock for the Troxler gauge.

The gauge was missing when the woman returned to her car on Saturday. According to Clark, ATEC officials initially did not notify the NRC of the disappearance because they assumed that the device had been picked up by a construction worker and put in a storage area.

When the device did not appear Monday, Clark said, ATEC called the NRC on Tuesday afternoon.

In Virginia, Troxler gauges and those who operate them are licensed by the NRC, Clark said. In about half of the states, the devices are regulated by state agencies.

Clark said a probe is under way to determine if ATEC was violating NRC rules, but "right now, we're more concerned with public health . . . . If this device was stolen, the person or persons who stole it could cause themselves severe damage."

Troxler gauges cost about $5,000, weigh about 25 pounds and contain a mixture of radioactive chemicals, police and NRC officials said. The device has a plunger that, when depressed, pushes two rods about a foot into the soil to measure moisture. The gauges bear signs warning of radiation danger.

Troxler gauges and similar devices have become commonplace around construction areas, where they are known as "nukes," said Phil Clark, a site manager with NVProperties, the builder of Oakton Grove. The gauges replaced the process of digging and measuring used years ago to determine if ground was solid enough, he said.

A spokesman for the maker said that 10 to 20 gauges had been stolen in the last year.

Officials said anyone finding the device should notify Fairfax County police or the NRC's operations center in Bethesda.