The devices could make people ill, and some, if they fell into the wrong hands, could be used to create weapons of terror, experts said.

The government created a new office this week to track down the devices.

The radioactive material could be used to make so-called "dirty" bombs — made by combining the radioactive ingredients and conventional explosives. Such a bomb would not create a nuclear explosion, but it could spread radiation and panic.

"In some cases… the quantities of radioactive material that are used in these devices is sufficiently large to cause injury or result in death," said Joel Lubenau, a former health physicist at the NRC.

"If they are used by terrorists, they can become a weapon of mass disruption," said Lubenau

"Just like a weapon, like a gun or a knife can be used, unfortunately, also radiation sources can be used as a weapon," said Fritz Steinhausler, a nuclear physicist at the Center for International Security and Cooperation at Stanford University.

Everyday Machines Contain Radioactive Materials

Thousands of machines that contain radioactive materials are used each day in U.S. industry and commerce, such as a soil density gauge that was stolen from a construction site in Howard County, Md., last March.

The gauge, used to measure how well the dirt is packed under roadways and pipelines, contains harmful radioactive material.

Similar material can be found in a broad array of equipment, including cancer radiation therapy machines, irradiation devices used to sterilize food, gamma ray cameras used in pipeline construction, and even unlikely devices such as a highway "exit" sign illuminated by radioactive material.

ABCNEWS requested that the NRC provide a list of the missing radioactive devices and their potential health risks. Citing security concerns, the agency declined.

Potentially Harmful

But the radiation in those devices can pose serious health risks.

In 1987, just 3 1/2 ounces of highly radioactive powder leaked from an abandoned radiology machine in Gioania, Brazil. Four people were killed, 60 hospitalized and thousands more were exposed to radiation.

Ten years later, a 16-year old New Jersey boy took home an "exit" sign from a construction site and cracked it open. In just 30 minutes, the boy was exposed to an amount of radiation humans normally receive over a three-month period. His home was quarantined and the clean up took more than two weeks, at a cost of $200,000.

How easy would it be for such equipment to fall into the hands of criminals? ABCNEWS found that one radioactive machine sat in a pawnshop in Riverdale, Md., for more than a year, where anyone could have bought it.