Obama orders a boost in security after terrorists threaten new strike

A terror threat against the United States set to coincide with the weekend's tenth anniversary of 9/11 may be traceable to al-Qa'ida and possibly to Ayman al-Zawahiri who assumed the network's leadership after the killing of Osama bin Laden, US anti-terror officials indicated last night.

Security was boosted in New York and Washington DC ahead of tomorrow's remembrance ceremony at Ground Zero in lower Manhattan to be led by President Barack Obama and his predecessor George W. Bush.

The US anti-terrorism apparatus was in overdrive trying to confirm details of the alleged conspiracy and ensure it is snuffed out. Vice President Joe Biden said US intelligence had identified a credible threat but it had not been corroborated or confirmed.

Other sources said the FBI believed it might involve the detonation of a car or truck bomb and three people were involved, including a US citizen. The authorities seemed unclear if the group was travelling to the US or had arrived.

"There are specifics, in that sense it was credible," Mr Biden told ABC News. "But there's no certitude. We don't have the smoking gun but we do have talk about using a car bomb."

He went on: "The thing we are all most worried about is what they call a 'lone ranger', a lone actor, not some extremely complicated plan like it took to take down the World Trade towers."

Secretary of State Hillary Clinton, told a criminal law conference in New York the fingerprints of al-Qa'ida appeared to be on the new threat. The US has "credible but unconfirmed reports that al-Qa'ida again is seeking to harm Americans and in particular to target New York and Washington," she said.

President Obama, who tomorrow will visit the Pentagon and Shanksville, Pennsylvania, the other two sites where hijacked planes came down, has ordered a redoubling of security.

Visible steps taken to increase security in New York included extra bag-checking patrols on the underground system and checkpoints at bridges and tunnels into Manhattan.