While most Americans focus on the threat of another aviation attack like the September 11 hijackings, the US government is quietly working to prevent something far worse — a catastrophic strike with a weapon of mass destruction.
Five years after the September 11 attacks killed nearly 3,000 people, Homeland Security Secretary Michael Chertoff has once again bolstered aviation security in a high-profile fashion in response to a failed plot in Britain to blow up US-bound planes.
But in a sign he fears other, more devastating attacks, Chertoff has also made his department focus on worst-case scenarios which could include nuclear or biological weapons.
Some analysts say the government is still not spending enough money to address such threats.
"An improvised nuclear device would be devastating with potentially hundreds of thousands of casualties. The damage would run in trillions of dollars," said Vayl Oxford, director of the department's Domestic Nuclear Detection Office.
"One of the concerns we have is that a weapon could be manufactured inside the US," Oxford said. His office is trying to protect against such a US-built weapon being brought into locations such as major cities where it could cause a great deal of damage.
The department this year began buying modernised equipment to scan cargo at ports and border cities for nuclear material—with equipment still in use at many sites.
Another concern is what experts fear is becoming a global nuclear arms race.
"That just increases the potential that the technology will find its way to terror organisations," said PJ Crowley, a spokesman for the US government and military for 28 years who is now director of national defence and homeland security at the Centre for American Progress.