Radioactive chunks of industrial metal that could be used in terrorist attacks are sometimes secured at sites around the country with no more than ordinary padlocks, according to congressional auditors. Also, auditors called ineffective the procedures for doing background checks of employers handling the materials.

A report by the Government Accountability Office, scheduled for release on Thursday by the Senate Homeland Security Committee, found that radiation material, some small enough to fit in a pocket, is often left where doors are unlocked or where skylights allow easy access. Four trucks carrying radioactive material have been stolen since December 2005, the report said, in some cases because a driver left the keys in the ignition or the door lock.

The report is a sequel to one produced two years ago that found lax handling of radioactive materials in hospitals. The concern is that a terrorist could use the stolen material in a so-called dirty bomb, an explosive that spews radioactive material in all directions, creating panic and contaminating a widespread area.

Hundreds of businesses around the country, many of them small, are licensed to hold thousands of “radiological sources,” which can be used for inspecting metals in a process that resembles an X-ray. Others use the materials for drilling for oil and gas, or for irradiating food to kill pathogens. The auditors visited 33 industrial sites in six states and found a variety of arrangements.