The investigators purchased a "small quantity" of radioactive materials from a commercial source, according to the GAO report prepared for Minnesota Republican Norm Coleman, chairman of the Senate Permanent Subcommittee on Investigations.

Nationally, at a cost so far of about $286 million, about 60 percent of all commercial goods entering the United States in containers by truck or ship and 77 percent of all private cars are now screened for radioactive material.

But flaws in the inspection procedures and limitations with the equipment mean that nuclear materials can possibly be sent illegally into the country through seaports or land borders, the GAO study found. And because the programme for installing radiation detectors is far behind schedule, many border crossing points, including many seaports, still have no detection equipment, the report says.

"We suffer from a massive blind spot in our cargo security measures," Coleman said in a statement that accompanied the report that was released Tuesday morning at a Senate hearing.

"It’s just an indictment of the system that it’s easier to get radiological material than it is to get cold medicine."

The test case
In the test case, undercover investigators posed as employees of a fictitious company and bought a small amount of radioactive material, most likely caesium, the New York Times said, though the GAO source did not specify what radioactive element was involved.

Then on December 15th 2005, they drove across the border at undisclosed locations from Canada and Mexico, intentionally picking spots where the detection equipment had been installed, American news outlets reported Wednesday.

The alarms went off in both locations, and the investigators were pulled aside for questioning. In both cases, they showed the agents from the Customs and Border Protection agency forged import licenses from the Nuclear Regulatory Commission, based on an image of the real document they found on the Internet, the New York Times reported.

The problem, the GAO report says, is that border agents have no routine way to confirm the validity of import licenses. The Nuclear Regulatory Commission, says the GAO, also improperly allows the sale of small amounts of radioactive materials without a permit. These are often substances that are used in industrial equipment and medical device, but that can also be used to create terrorist weapons.

David McIntyre, a Nuclear Regulatory Commission spokesman, disputed the claim by the Congressional investigators that the amount of material bought and taken across the border would have been enough to build a dirty bomb in interview with US media.

Dirty bombs, or radiological dispersal devices, are envisioned as being made from a radioactive substance used in tandem with conventional explosives. Though they do not have the yield of a proper nuclear device, they can force long-term evacuation by spreading low levels of radioactivity across an area after detonation.

But McIntyre said he agreed that Customs officials at the borders must be able to confirm quickly the validity of import licenses.

"We are working with Customs and the Department of Homeland Security to make sure this information is available to them twenty-four seven," he said.

The investigation, part of a three-year inquiry by the Senate Homeland Security and Governmental Affairs Committee into the nation’s vulnerability to nuclear smuggling, particularly at ports, found many other weaknesses in the radiation detection campaign.

The GAO reports
The focus of the three GAO reports at the hearings beginning on Tuesday will be on what the federal government has done to protect the country against nuclear terrorism. This weeks hearings come after almost three years of bipartisan and joint senate and congressional investigations into the subject.

A second GAO report notes that while the departments of State, Energy and Defence have provided radiation-detection equipment to 36 countries since 1994 to combat nuclear smuggling, operating the equipment has proven challenging.

Those challenges include technical limitations of some of the equipment, a lack of supporting infrastructure at some border sites and corruption of some foreign border security officials.

The second report also notes that the State Department, the lead interagency co-ordinator in this effort, has not maintained a master list of US-funded radiation-detection equipment in foreign countries.

Without such a list, program managers at the agencies involved "cannot accurately assess if equipment is operational and being used as intended; determine the equipment needs of countries where they plan to provide assistance; or detect if an agency has unknowingly supplied duplicative equipment," the report says.

It further criticises the State Department, saying that "without taking steps to ensure that all previously provided radiation-detection equipment, specifically hand-held equipment, is adequately maintained and remains operational, State cannot ensure the continued effectiveness or long-term sustainability of this equipment."

A third GAO report observes that, while the Department of Homeland Security has made progress in deploying radiation-detection equipment at US ports—which include 670 portal monitors and more than 19,000 pieces of hand-held radiation detection equipment as of last December—the agency’s program goals are "unrealistic" and its cost estimate is "uncertain."

How the equipment works
The primary radiation monitors, which look like a standard tollbooth, cannot distinguish between naturally occurring radiation, and radioactivity in bomb-making substances.
Yet when Customs agents use hand-held radiation devices, which are supposed to clear false alarms by isolating the specific type of radiation, the standard procedure is to walk along the exterior of the container, rather than opening it. Because of this, the hand-held devices can produce unreliable results, the New York Times reported.

Installation of the radiation screening equipment is running behind schedule, largely because of delays in appropriating federal money, problems figuring out how to use the devices to screen rail cars and disputes with ports that are worried about slowing the movement of goods, the GAO report says.

Investigators predict that the project—which the Homeland Security Department estimates will cost $1.3 billion—is going to cost much more.