Frank Gardner BBC Security corresondent

Security across the United States has been tightened for the first anniversary of the 11 September 11 attacks.

But throughout the year the US authorities have been trying to plug the gaps in security exposed by the biggest attack on the US mainland in history.

Although no specific plot has been identified in the US itself, Washington has reportedly allocated an extra $3.5bn for emergency response teams.

Mass destruction

At a military exercise in Washington soldiers prepare for a nightmare, a poison gas attack on the Pentagon. This was just an exercise but the US military fears that one day it could actually happen.

If you stay off the airlines you can move around anywhere you like, you don't have to register with the police anywhere

Col Patrick Lang

"We have to recognise that terrorist networks have relationships with terrorist states that have weapons of mass destruction, and that they inevitably are going to get their hands on them and they will not hesitate one minute in using them", US Defence Secretary Donald Rumsfeld explained in a previous statement on the continuing threat.

The US capital never used to feel vulnerable to attack. But after a hijacked plane crashed into the Pentagon, that changed last September.

Official rethink

Over the last 12 months the US administration has had to think the unthinkable - that al-Qaeda, or its affiliates, may try to set off a weapon of mass destruction right here in the heart of Washington.

So US security has been raised to historically high levels. But unfortunately for America, the threat is likely to remain.

Training for terror has become routine for the FBI. Its agents are now co-operating more closely with the CIA than a year ago.

More than 1,200 people have been detained in the US. But, in this vast country, there are many reasons why the nature of the terror threat is still hard to identify.

Colonel Patrick Lang is a former officer with the Defence Intelligence Agency.

"The nature is greatly increased by the fact that we're such an open society, and in fact there are no internal movement controls inside the United States," he said.

Donald Rumsfeld: Warning of future dangers

"If you stay off the airlines you can move around anywhere you like, you don't have to register with the police anywhere, and aliens resident in the United States are given exactly the same legal status as American citizens. So it's easy to operate in a place like this."

In a paper exercise this year, a Washington think-tank looked at what would happen, if a so-called 'dirty bomb' - radioactive material wraped in conventional explosive - was detonated in the middle of the city.

David Harman is a scientist with the Centre for Strategic and International Studies in Washington and warns of the potential danger from such an attack.

"The radiological element, depending on the amount of radiation and the weather patterns - you would have dispersal over this whole area and you would probably need to decontaminate the people and the places here," he said.

Screening cargo for radioactive materials is just one of the new security measures being introduced in the US.

Many others are being kept secret. The authorities know they can never guard completely against a determined, and unseen foe. All they can do, is to reduce their enemy's chances of success.