Washington and New York on alert for al-Qaeda attack

New York and Washington were on high alert on Friday night amid fears that Ayman al-Zawahiri, the new al-Qaeda chief, may be seeking to avenge Osama bin Laden's death by launching a terrorist attack around the anniversary of September 11.

Joe Biden, the US vice-president, said intelligence had "specific" information from a "credible source" that a plot involving "a car bomb" may be under way.

However, he stressed: "We don't have a smoking gun."

Hillary Clinton, the secretary of state, named al-Qaeda as being behind the suspected threat.

It came as The Daily Telegraph learned that MI5 has been advising New York authorities on security. It is understood that Security Service officials have given briefings on their experience of dealing with IRA vehicle bombs in London.

Both cities were increasing security ahead of ceremonies in commemoration of the 10th anniversary of the attacks, which killed almost 3,000 people. President Barack Obama and former president George W Bush are to attend a ceremony at Ground Zero in Manhattan on Sunday.

Police checkpoints were in place on Manhattan streets. Officers were increasing numbers on New York's transport network, making random checks on vehicles and bags, and monitoring airports, bridges and tunnels.

Michael Bloomberg, the mayor of New York City, said: "The NYPD, FBI and entire intelligence community have been on heightened alert."

It is thought that three people who entered the US by air last month, one of them a US citizen, are urgently being sought in relation to the suspected threat. However, their identities were not thought to be known on Friday night, and officials continued to search arrivals lists.

Notes seized during the May raid on Osama bin Laden's compound in Pakistan, in which he was shot dead, raised the possibility of a 9/11 anniversary attack, officials said.

Janice Fedarcyk, the head of the FBI's New York unit, said: "Al-Qaeda has shown an interest in important dates and anniversaries."

CIA officials reportedly believe that those involved embarked on a journey to the US from the tribal areas of Pakistan, where several important al-Qaeda and Taliban operatives, including Zawahiri, are thought to be based. Multiple reports said officials had "reason to believe" Zawahiri was involved.

In a video message released three weeks ago, Zawahiri told followers: "America today is staggering. Hunt her down wherever you may encounter her. Hunt her down to cut what is left of her corruption's tail."

Faisal Shahzad, the 32-year-old Pakistani-American who tried to detonate a car bomb in New York's Times Square last year, is believed to have received training in the tribal areas. Eric Holder, the US attorney general, said the Pakistani Taliban were "intimately involved" in his plot.

In a speech in New York, Mrs Clinton said the unconfirmed report suggested "al-Qaeda again is seeking to harm Americans". She promised a "relentless" campaign against it and said a "Global Counter-Terrorism Forum", bringing together international officials, would be established later this month.

Bill Baroni, the deputy head of the Port Authority of New York and New Jersey, which owns the site, told The Daily Telegraph that British intelligence officers had been helping to devise its security. Officials were "learning from the experience that London has had".

David Samson, chairman of the authority's board of commissioners, established a relationship with MI5 in a visit to London as New Jersey's attorney general. "They have been dealing with [terrorism] for 30 years and have an extremely sophisticated system that works very well," he said.

Ceremonies are also to be held on Sunday at the Pentagon, in Virginia, where 184 people died when American Airlines flight 77 struck, and in Shanksville, Pennsylvania, where all 40 passengers and crew members died when United Airlines flight 93 was flown into a field.

A ceremony to commemorate the 343 New York firefighters killed in the attacks is due to be held on Saturday in New York City.