US terrorism experts are warning that more work needs to be done to avoid further attacks by extremists. The warning comes as the second anniversary of September 11 draws near. The US Department of Homeland Security said it was confident the country was now protected like never before.

But Matthew Devost, the head of the Terrorism Research Center, a private research firm, said it would be some time before the Homeland Security Department as a fully functioning, integrated unit.

He said: “We always use the analogy of trying to build an aeroplane during take off.

“They’re taking all these pieces and components, trying to integrate them into one department and still at the same time effectively increase the level of homeland security within the United States.

“Our perception of time is a lot shorter than the threat’s perception of time.

“We think two years is a long time, they think five years is a short period of time and they’ll outwait us so that the more complacent we become the greater the risk that we pose to ourselves.”

Terrorism analyst Brian Jenkins said he had no doubt that the next big terrorist attack will happen on US soil.

He said al-Qaeda was a patient terrorist group and warned Americans not to feel consoled by the fact that two years has passed since the September 11, 2001 attacks.

“Planning for 9/11 began three – four years before the attack,” he said.

“So the fact that nothing has occurred is not an indicator – and certainly doesn’t provide any comfort – that nothing will occur.

” We could see something a month from now, a year from now, three years from now.”

Former Republican Senator Warren Rudman, who warned of US vulnerability to terrorist attacks before September 11, 2001, said further attacks were inevitable.

In the wake of September 11 the US government reorganised itself with the creation of the Homeland Security Department which combined nearly two dozen agencies and 170,000 staff.

But state and local officials said they had not received training and equipment they needed to respond to terrorist attacks, particularly those involving chemical or biological weapons.