The Government Accountability Office released the report in advance of Senate Homeland Security subcommittee meetings scheduled to begin Tuesday.
The report, commissioned by Committee Chairman Norm Coleman, R-Minn., described a December 2005 test during which two teams of investigators posing as employees of a fake company passed through ports at the U.S.-Mexico and U.S.-Canada borders.
Though radiation monitors installed by the Department of Homeland Security detected the material carried by the investigators, border patrol agents failed to recognize forged documents authorizing the materials' entry.
"The CPB (Customs and Border Patrol) inspectors never questioned the authenticity of the investigators' counterfeit bill of lading or the counterfeit NRC (Nuclear Regulatory Commission) authorizing them to receive, acquire, possess and transfer radioactive sources," the GAO said in a letter to Coleman, according to Reuters.
The amount of materials the investigators carried was enough to make two so-called "dirty bombs," devices that could discharge enough radioactive material to affect an entire neighborhood.
The investigators purchased small amounts of the material from a commercial source. The company did not question the purchase because laws do not require suppliers to determine the legitimacy of sales of radioactive material in small quantities, the report said.
Preventing a dirty bomb attack has been a major priority of the Bush administration since the Sept. 11, 2001 terrorist attacks on the United States.
"Although our systems worked and our officers appear to have followed our established protocols for radiological alarms, the bottom line is that raw material was allowed into our country using fictitious Nuclear Regulatory Commission documentation," U.S. Customs and Border Protection, the office in charge of security operations at U.S. ports, said in a statement.
"CBP is strengthening this process, working with the NRC, to ensure that we have the ability to validate NRC licenses and documentation through our National Targeting Center 24 hours per day, seven days a week," the statement read.
The Department of Homeland Security has installed about 740 radiation detection devices at domestic ports in an effort to prevent illegal materials entering the country, a CBP spokesman said.
A spokeswoman for the Department of Homeland Security said 80 percent of private vehicles and 90 percent of commercial trucks at the northern border are screened and at the southern border, 74 percent of private vehicles and 88 percent of trucks.
The department screens only 51 percent of containers entering the country at seaports.
But plans to install an additional 2,400 monitors have fallen behind schedule and border patrol agents have had problems operating the monitors, the GAO report said. The CBP said the department is fully funded to install the remaining monitors by 2010.
Outside the United States, the departments of State, Energy and Defense have provided 36 countries with radiation-detection equipment to help stop illegal material at its source, a second GAO report said, according to CNN.
"We suffer from a massive blind spot in our cargo security measures," Coleman said in a statement responding to the report, according to the New York Times.
Tuesday's Senate hearings will focus on what the government is doing to protect the country against nuclear terrorism.