WASHINGTON, March 28: The United States has not adequately protected itself against the nightmare scenario of a nuclear attack by terrorists, the head of the panel that investigated the Sept. 11, 2001, attacks said on Tuesday. Former New Jersey Gov. Tom Kean told a Senate panel that neither the Bush administration nor Congress has done all it can to protect American lives and the U.S. economy: “The size of the problem still totally dwarfs the policy response.”

Another expert, Stephen Flynn of the Council on Foreign Relations, outlined numerous security gaps in global trade and shipping. Flynn, a retired Coast Guard commander, said the country is living on “borrowed time” for avoiding a so-called dirty bomb that could contaminate a financial district or residential neighborhood and wreak havoc on the economy.

They were among the witnesses before the Senate Permanent Subcommittee on Investigations one day after the release of a government report that said that four years after the attacks on New York, Washington and Pennsylvania, federal undercover investigators were able to enter the United States with enough radioactive material for two dirty bombs.

Radiation monitors did detect the radioactive materials in investigators’ cars crossing the U.S.-Mexican and U.S.-Canadian borders in December 2005, but border patrol officials did not realize the shipping documents and permits were forged, the Government Accountability Office said in that report.

The commission disbanded in 2004 after conducting a comprehensive probe of the attacks. But its 10 members have remained active in promoting better national defense.

A dirty bomb could kill people in the blast’s immediate vicinity and spew radioactive material, making an area unusable. It could cause immense economic disruption, as other ports and transport routes could be shut down temporarily, just as air traffic was halted after the 2001 attacks.

“We’re not acting like a nation at war,” said Flynn, adding that security plans have not drawn on all the available technology nor the incentives in the private sector for closing security gaps.

Sen. Carl Levin, a Michigan Democrat, noted that a nuclear bomb would be far more devastating than a dirty bomb.

—Reuters