New York and Washington on alert over 9/11 al-Qa'ida threat

Police check cars in Manhattan as security is tightened (Getty Images)

A terror threat against the United States planned to coincide with this 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 terror officials indicated last night.

Security precautions were being ramped up in New York and Washington DC yesterday as preparations were under way for tomorrow's remembrance ceremony at Ground Zero in lower Manhattan to be led by both President Barack Obama and his predecessor George W. Bush.

Meanwhile the whole anti-terrorism apparatus of the US that was hugely scaled up in the wake of 9/11, including large sections of the FBI, the CIA as well as city police departments, was in overdrive last night trying to confirm details of the alleged conspiracy and ensure that it was snuffed out in time.

Vice President Joe Biden confirmed that US intelligence has identified a threat that was considered credible but which 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. It didn't seem the authorities were clear if the group was travelling to the US or had already made it in.

"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."

The Secretary of State, Hillary Clinton, told a criminal law conference in New York that the finger-prints of the al-Qa'ida appeared to be on the new threat. The US has "credible but unconfirmed report 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 also travel to the Pentagon and to Shanksville, Pennsylvania, the other two sites where hijacked plans came down, ordered a redoubling of security in the wake of the new terror warning.

There has been no loss of life to terrorism on US soil since 9/11. Attacks have been attempted, however, including a Christmas Day bid to down an airliner flying to Detroit from Amsterdam nearly three years ago and a botched car bomb plot to devastate Times Square. The latter was foiled when a street vendor became suspicious of the car. While anti-terrorism units have identified possible threats aimed at the US over the last decade, the weeks leading up to this weekend's anniversary had apparently seemed quiet until this warning surfaced.

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. At rush hour yesterday all but one of the inbound lanes on the Brooklyn Bridge were choked by roadblocks. Mayor Michael Bloomberg urged New Yorkers to carry on as normal but to be vigilant. "The best thing we can do to fight terror is to not let it intimidate us," he said.

There will be six moments of silence at the Ground Zero ceremony tomorrow marking when the two airliners struck the Twin Towers and when they fell down but also the exact moments when the other two planes involved in that day's tragedy crashed at the Pentagon and in Pennsylvania. Tomorrow will also be the first time that families of the victims will see the new memorial at the World Trade Center site.

The day of remembrance will finish with a memorial concert in the Kennedy Centre in Washington where Mr Obama will also speak. The White House said none of his travel plans had so far been affected by the terror threat.

Privately US officials will say that there has been concern inside the US government for some time about the danger of another attack this weekend, not least because of information found in a notebook in the compound where bin Laden was killed that indicated a desire to strike at the US on important dates, including the tenth anniversary of 9/11. Additionally, al-Qa'ida may be motivated by a desire to avenge the killing of bin Laden in May this year.