US senators have urged the government to act more quickly to strengthen border security after radioactive material was brought into the country.
Radiation detectors are being installed at US ports of entry
The quantity of Cesium-137 was said to be enough to build two dirty bombs.
The Senate Homeland Security Committee began hearings on Tuesday to discuss an undercover operation by US agents.
They crossed into the US from both Canada and Mexico with the material, despite radiation detection alarms going off when they went through.
Border guards let them through after being shown false paperwork.
At the opening of the Homeland Security Committee hearings, lawmakers described the incident as "an alarming wake-up call".
"If terrorists were to obtain nuclear or radiological material and smuggle it into this country, the consequences could be catastrophic," said Republican Senator Susan Collins, the chairwoman of the panel.
The Government Accountability Office (GAO) said in its report on the incident: "The [Customs and Border Patrol] inspectors never questioned the authenticity" of the documents shown.
The radioactive material was bought from a commercial source by telephone, the GAO said.
Vendors are not required to ask about or check a purchaser's documentation when small quantities are purchased, the agency said.
The GAO said the radioactive material was enough for two dirty bombs - devices that use conventional explosives to spread dangerous radiation over a wide area.
It also found that the installation of 3,034 radiation detectors at US border crossings, seaports, airports and mail facilities was taking longer and costing more than anticipated.
"This operation demonstrated that the Nuclear Regulatory Commission [NRC] is stuck in a pre-9/11 mindset in a post-9/11 world and must modernise its procedures," said Republican Senator Norm Coleman, the chairman of the Senate committee, who ordered the investigation.
However, NRC spokesman David McIntyre defended his body and disputed that there was enough material to make two bombs.
"It was basically the radioactive equivalent of what's in a smoke detector," he told the Associated Press news agency.