The investigators from Congress's Government Accountability Office (GAO) said the sting operation showed that US security measures remain inadequate despite new policies designed to prevent radioactive material from falling into the hands of terrorist networks.

The investigators easily secured the license from the Nuclear Regulatory Commission within 28 days after a minimal background check without an interview or a visit to the purported company, the Washington Post reported, citing the GAO report to be released later Thursday.

The GAO agents ordered portable moisture density gauges, which contain radioactive americium-241 and cesium-137 and are often used at construction sites to assess soil, water and pavement, the paper said.

The investigators were able to alter the license, removing a limit on the amount of radioactive material they could buy, the report says.

"We altered the license so that it appeared our bogus company could purchase an unrestricted quantity" of radioactive material, the Post said, citing the report.

"Given that terrorists have expressed an interest in obtaining nuclear material, the Congress and the American people expect licensing programs for these materials to be secure," said Gregory Kutz, an investigator at the accountability office, in testimony prepared for a congressional hearing.

Counter-terrorism experts have warned of the threat posed by a dirty bomb, which uses conventional explosives as well as radioactive material to contaminate a wide area.

The investigators called off the operation before the devices were delivered, The New York Times reported.

A previous undercover operation purchased enough radioactive material abroad to make two dirty bombs and smuggled them into the United States at two points, one on the Canadian border and one on the border with Mexico.

The GAO report is due to be released at a Senate hearing on Thursday.