Police have admitted that they are no nearer to finding a mastermind behind the July 7 bombings, as Scotland Yard responds to a new warning that terrorists plan to drive hijacked fuel tankers into petrol stations.

A focus for the July 7 investigation will be to trace any video evidence recorded by the four bombers admitting the attacks, as other British suicide bombers have done in the past.

Counter-terrorism officers want to pour more manpower into investigating the July 7 atrocity, but will now have to concentrate on the weekend warning from the US Department of Homeland Security, which said that terrorists plan to strike within the next month in Britain and in New York, Chicago and Los Angeles.

Scotland Yard’s priority remains preventing another attack but it will draft more officers into hunting the support network behind the 7/7 attacks, which killed 56 people.

Police do not know where the four bombers spent the night before the attack and senior officers do not believe that they acted alone.

One senior source said: "The background of these four does not suggest that they could have done everything themselves. Someone had to train them in building a sophisticated set of bombs and all sorts of other logistical support."

Sir Ian Blair, the Metropolitan Police Commissioner, has conceded that tracing any third terrorist cell is proving difficult as there is no intelligence on those involved in the attacks.

Scotland Yard dare not relax the London lockdown with reports that another terrorist cell is in place. Sir Ian said at the weekend: "The fact that we had two attacks makes the possibility of a third attack more likely, not less likely."

US agents would not say how they had discovered details of the plot to hijack fuel tankers, a favoured terrorist tactic in the Middle East.

British police are understood to have foiled a number of plots involving lorry bombs, including one on London pubs and nightclubs last year.

The Department of Transport has issued security guidelines for fuel companies and others with fleets of tankers. Transport experts have called for closer monitoring of tankers on the road and companies are examining technology to allow vehicles to be shut down if they are hijacked.

Al-Qaeda figures in US custody have said that videos showing how to prepare lorry bombs and footage of tankers ramming buildings are shown at training camps. Khalid Sheikh Mohammed, one of al-Qaeda’s leading tacticians who was arrested in 2003, has reportedly told interrogators that he drew up plans to target petrol stations, because of "their apparent vulnerability and the potential destructive force of a fuel-driven explosion", the Homeland Security report says.

It adds: "Al-Qaeda leaders plan to employ various types of fuel trucks as vehicle-borne improvised explosive devices in an effort to cause mass casualties . . . The stated goal is the collapse of the US economy."

Today both the Department for Homeland Security and the FBI played down the threats outlined in the report, saying that the warning was from a single, unreliable source and that it did not refer to London.

"The information is uncorroborated at this time and the source of the information has questionable reliability. The source has not been deemed reliable," Katie Montgomery, the spokeswoman, told the Reuters news agency.

"At this time the Department for Homeland Security does not have any specific credible information that suggests a terrorist attack in the US is imminent," she added.

FBI spokesman Steve Kodak said the report referred to Chicago, LA and New York, and did not specifically refer to London.

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