Senior Pakistani security officials told The Times that Mohammad Sidique Khan, Shehzad Tanweer and Hasib Hussain — all from Leeds — met known al-Qaeda suspects during their trip. They spent most of their time not in religious schools but in the company of figures from outlawed militant groups.

The officials said: “Khan and Tanweer travelled together to Pakistan in November 2004 and stayed through until early 2005. They were joined by Hussain.”

Pakistani police are also searching for 12 Britons whose whereabouts are unknown and whose names have emerged in an urgent review of intelligence after the attacks.

Unpicking the Pakistani arm of the British al-Qaeda network is seen as the key to the London investigation and preventing further bomb attacks.The toll from the July 7 bombings now stands at 55 but is expected to rise; 47 of the dead have been formally identified.

The Pakistani authorities have arrested more than a dozen suspects over the weekend in connection with the bombings and with possible links to Khan, 30, Tanweer, 22, and Hussain, 18.

Telephone contacts between the bombers and Pakistan are being closely analysed.

Investigators in London and Islamabad are in constant communication but the suspected mastermind of the London bombings continues to elude them.

The 33-year-old Briton entered Britain at Felixstowe on a ferry from the Netherlands or Belgium a fortnight before the bombings. He flew out from Heathrow hours before his recruits embarked on their deadly missions.

Inquiries into the background of the other bomber, the Jamaican-born Jermaine Lindsay, 19, have taken British police in a different direction.

Former schoolfriends of Lindsay, who grew up in Huddersfield, said that he visited Afghanistan four years ago and returned to Britain as a hardline Muslim. Scotland Yard sources said last night that officers were looking closely at Lindsay’s connections in the Luton area.

One officer said that Lindsay appeared to have links with criminal activity in the town.

Luton has also been a hotbed of radical Islamist activity for several years, with extremist groups such as al-Muhajiroun having an influence over young Muslims there. Last year several arrests were made in the town in connection with another anti-terrorist operation. Details of that cannot be reported because it is the subject of a forthcoming criminal trial.

According to British and American intelligence sources, investigators conducting that operation are believed to have encountered the names of Lindsay and Khan “on the periphery” of the inquiry.

Luton was also the setting off point for the four bombers. They were captured on CCTV at about 7.20am. They boarded a train to London, and disembarked at King’s Cross.

Khan detonated his bomb at Edgware Road, Tanweer at Aldgate and Lindsay on a Piccadilly Line train at King’s Cross. The three Tube bombs exploded at 8.50am. Hussain set off his rucksack bomb on a No 30 bus at Tavistock Square almost an hour later.

The bombers left behind a cache of high-explosives and bomb components that could have been used to make at least three or four more devices.

Detonators and enough of the acetone peroxide-based explosive to make two bombs were found in Lindsay’s car at Luton, along with a gun and ammunition, a senior anti- terrorist source said.

At one of the properties being searched in Leeds, policehave recovered enough explosives to make two devices similar to those used on July 7.

The discovery of the weapons and explosives poses a series of difficult questions for those leading the investigation, who say they are pursuing a wide range of inquiries.

Detectives have to consider whether there may be a second bomb team equipped with further explosives and ready to strike again.

They are also examining the possibility that some or all of the London bombers did not know they were taking part in a suicide attack and thought they would be returning to the car.

The investigation team is still trying to determine where the bombers spent the night before the attacks.

The inquiry is the country’s biggest criminal investigation will stretch anti-terrorist and intelligence resources. According to The Sunday Times, MI5 made “a quick assessment” of Khan last year but judged that he was not a threat to national security. Police were not involved in that process.

The paper reported a senior government official saying: “MI5 is fair game at the moment. We’ve only got finite resources. You can only concentrate resources on those people who are a direct threat to national security.”

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