Detectives hunting the West End car bombers believe the suspects are most likely to be home-grown extremists linked to an overlapping network of terrorist cells implicated in previous plots against British targets.

Some may be known to police and be on the run after escaping Home Office control orders.

Those in the frame may be associates of the so-called Crevice gang, which planned to attack the Ministry of Sound nightclub in central London and the Bluewater shopping mall in Kent.

Members of the five-man cell, who were jailed for life in April, were directed by “core” Al-Qaeda figures after training in terror camps in Pakistan.

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BACKGROUND

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• Men in bungled raid say police attacked again

• More than Vigilance

• 'Dirty bomb' mastermind plotted wave of atrocities across Britain

• Jihadi house parties of hate

The brother of one jailed gang member, who has been on the run since breaching a government-imposed control order six weeks ago, is said to have been keen to bomb a nightclub.

“There is a real possibility the suspects may have a connection through a family of cells with the Crevice gang,” said a senior government security official. “It is very possible these people met each other at training camps.”

The suspects may also have drawn inspiration from another cell led by Dhiren Barot, an Al-Qaeda “general”, who drew up sophisticated plans to target London hotels and office buildings by parking limousines packed with gas canisters in underground car parks.

Barot, now serving 30 years in prison, outlined his plot in a document called Gas Limos Project, which he prepared for Al-Qaeda leaders in Pakistan.

In the document, which was found on a laptop computer, Barot wrote: “Place 12-13 full size cylinders in each limo. A few should be sprayed yellow because yellow cylinders in the West signify toxic gas.

“This will aid to spread terror and chaos when the emergency service teams arrive. Underneath and around the cylinders generously place some loose pieces of charcoal (that have been presoaked in petrol). Place a 10-litre petrol can containing nails next to each cylinder.”

Barot envisaged detonating the cars with grenades or pipe bombs. “Estimated casualties to be hundreds if the building collapses,” he said.

Security officials insist there was no intelligence pointing to a car bomb attack in the West End. But there are concerns that extremists who were on the surveillance back-burner could have escaped their attentions.

“They are saying this is leftfield, that it came out of the blue,” said a senior Whitehall official. “What that means is they think it’s possible that these were people they have been aware of who suddenly did this.

“It may be that these are people that they know about – but just hadn’t realised what they were up to.”

Patrick Mercer, the Tory MP and security expert, said: “The real nervousness for the agencies is that these may be people they know but haven’t picked up. It’s happened before.

“It calls into question the strategy about leaving these people in play and not arresting them.”

Such concerns reflect the fall-out from the investigation into the July 7 attacks two years ago, which killed 52 people.

The authorities initially claimed the suicide bombers were unknown “clean skins”, but it soon emerged that Mohammad Sidique Khan, the 7/7 leader, and Shehzad Tanweer had been under surveillance a year earlier.

The two bombers were photographed at meetings with Omar Khyam, the leader of the Crevice gang that was plotting to detonate a fertiliser bomb.

Bugged conversations of the Crevice cell revealed the plotters’ disdain for nightclubs. Discussing the Ministry of Sound, one gang member said: “No one can put their hands up and say they are innocent...those slags dancing around.”

A key member of the Crevice gang was Anthony Garcia. During his trial, an Al-Qaeda supergrass revealed that Garcia’s brother, Lamine Adam, had allegedly wanted to bomb a nightclub and was seeking a formula for explosives.

The supergrass’s testimony was not considered strong enough for prosecution. However, Adam, 26, and his younger brother, Ibrahim, 20, were placed on control orders in February 2006 on the grounds that they planned to kill British soldiers serving abroad.

The two brothers and a friend, Cerie Bullivant, 24, who was put on a control order last July, went on the run six weeks ago. Police think they may have slipped abroad, but they cannot rule out that the trio could still pose a threat within the UK.

Lord Carlile, the government’s terrorism watchdog, said: “I would certainly not view this as a failure by the authorities in any sense,” he said. “Looking for home-grown cells is like looking for a needle in a haystack.”

Baptism of fire for security trio

It was a baptism of fire for Britain’s new anti-terror team.

Jacqui Smith, the home secretary, was woken by an early morning phone call on Friday alerting her to the attempted attack. She chaired a meeting of the Cobra emergency response committee in her first full day in the job.

Admiral Sir Alan West, 59, the former head of the Royal Navy, is the Home Office security minister. A hero of the Falklands war where his frigate, HMS Ardent, was sunk in an Argentine attack, he was awarded the Distinguished Service Cross.

Lord Stevens, 64, former commissioner of the Metropolitan police, is the government’s new adviser on international security. Best known internationally for leading the Operation Paget inquiry into the death of Diana, Princess of Wales, in 1997, which concluded it was an accident.

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