Senators said Tuesday the United States must act more quickly to strengthen security at U.S. entry points following the disclosure that undercover investigators breached security by slipping nuclear material into the United States.

"There are several gaps in our defense," Sen. Norm Coleman, R-Minn., said, opening a Senate Homeland Security subcommittee hearing on the matter. "These issues must be addressed with a sense of urgency."

Sen. Carl Levin of Michigan, the panel's top Democrat, called the outcome of the Government Accountability Office's investigation "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: a tremendous loss of life and a crippling blow to our economy," said Sen. Susan Collins, R-Maine, the chairwoman of the full Homeland Security Committee.

In a test last year, the small amounts of cesium-137, which is used in industrial gauges, triggered radiation alarms in Texas and Washington state. The material was enough to make two small "dirty bombs," officials said, yet U.S. customs agents permitted the investigators to enter the United States after being tricked with counterfeit documents.

The Bush administration said Tuesday that within 30 days it will give U.S. Customs and Border Protection agents the tools they need to verify such documents in the future.

In a series of reports, the GAO, which is the investigative arm of Congress, found that the Homeland Security Department's goal of installing 3,034 radiation detectors by September 2009 across the United States — at border crossings, seaports, airports and mail facilities — was "unlikely."

Investigators also said the government probably will spend $342 million more than it expects to complete the job, given its current costs and pace. Between October 2000 and October 2005, they said, the government spent about $286 million installing radiation monitors inside the United States.

The commission, in charge of overseeing nuclear reactor and nuclear substance safety, defended its record.

"Security has been of prime importance for us on the materials front and the power plant front since 9/11," commission spokesman David McIntyre said.

Vayl Oxford, who heads the Homeland Security Department's Domestic Nuclear Detection Office, said Monday that the substance could have been used in a radiological weapon with limited effects. On Tuesday, he told lawmakers that his comment was not meant to discount the psychological and economic impact a dirty bomb could have.

Jayson Ahern, the assistant customs commissioner for field operations, said a system for U.S. customs agents to confirm the authenticity of government licenses will be in place within 30 days.

To test security at U.S. borders with Mexico and Canada, GAO investigators last year represented themselves as employees of a fake company and obtained cesium-137.

They attempted to cross into the United States with the substance — enough to possibly create two crude radiological bombs that could spread radiation if spread by the blast of a conventional explosive.

When stopped, the investigators presented counterfeit shipping papers and NRC documents that allegedly permitted them to receive, acquire, possess and transfer radioactive substances.

Investigators found that customs agents weren't able to check whether a person caught with radioactive materials was permitted to possess the materials under a government-issued license.

"Unless nuclear smugglers in possession of faked license documents raised suspicions in some other way, CBP officers could follow agency guidelines yet unwittingly allow them to enter the country with their illegal nuclear cargo," a report said. It described this problem as "a significant gap" in the nation's safety procedures.

False radiation alarms are common — sometimes occurring more than 100 times a day — although the GAO said inspectors generally do a good job distinguishing nuisance alarms from actual ones. False alarms can be caused by ceramics, fertilizers, bananas and even patients who have recently undergone some types of medical procedures.