The attacks of 9/11 triggered the most sweeping overhaul of US intelligence and law enforcement for half a century with the creation of the Department of Homeland Security, formal co-operation and information-sharing among 15 intelligence agencies and a change within the Federal Bureau of Investigations (FBI) to prioritise counter-terrorism.
Tens of thousands of security personnel have been hired. The National Security Agency, the biggest and most secretive US intelligence agency - has records of the origin and destination of every phone call made in the US and can trawl through e-mails and other communications.
Yawning security gaps remain, however, as the 9/11 Commission warned in a report issued late last year. They include the fact that airport security screeners don't check all passengers and their carry-on baggage for explosives, nor does the government fully inspect airline cargo. The Bush administration has spent more than $10 billion (€7.9 billion) improving port security since 9/11 but only a tiny fraction of containers shipped into the US are inspected. And as Hurricane Katrina showed, emergency workers can still find their communications cut off because there is no dedicated wavelength for first responders.