Millions of pounds have been spent, detectives have travelled to a dozen countries and questioned thousands of people but they have yet to find anyone who played a role in the attacks that killed 52 innocent people.

Peter Clarke, the senior Scotland Yard officer in charge of anti-terrorist operations, admitted yesterday that police were still searching for friends in whom the bombers may have confided.

They do not know who first radicalised the four bombers, nor who encouraged them to take part in the first suicide operation in western Europe.

There have been a handful of arrests in Britain and abroad, but police concede that they are looking for evidence of extremist leaders who could have masterminded the attack and any militant network that provided money, accommodation or bomb-making skills.

The official report into the attacks claims that the four men acted alone. Many of the victims’ families and survivors contest this and are petitioning for an independent inquiry into what the authorities knew about the bombers.

Senior officers admit that many vital questions remain unanswered. So far, the investigation has collected 29,500 exhibits and 6,000 hours of CCTV footage in the search for clues. Police have taken 13,353 witness statements from survivors, other passengers and people who knew the bombers.

Speaking days before the first anniversary of the attack Mr Clarke, the deputy assistant commissioner in charge of the anti-terrorist branch, said that forensic tests and checks on the CCTV were still going on. He did not rule out arrests and charges over 7/7 but said that no warrants had been issued.

“We need to know who else,apart from the bombers, knew what they were planning. Did anyone encourage them? Did anyone help them with money, accommodation or expertise in bomb-making?” Mr Clarke said: “People are not going to come forward and say they knew about it. The four core criminals have died and so we trying to backtrack to what surrounded it.”

The four acted normally before the attacks, Mr Clarke said. “The families were completely unaware of what was happening.”

The news of the failure to find any real clues in the 7/7 case came as MI5 announced that it had foiled an attempt by a number of British al-Qaeda supporters to join the Security Service. Security sources said that evidence had emerged of several individuals with Islamic militant backgrounds who had shown interest in applying to work for MI5.

MI5 is currently boosting its workforce by 50 per cent in a major recruiting campaign, and is looking for suitable candidates for the counter- international terrorism branch. In particular, the organisation is looking for Arabic, Farsi, Urdu and other linguists.

Sources said that a number of applicants with associations with known Islamic militants had been “weeded out” before they could get any further down the recruiting stages.

The security sources also said that MI5 was keeping watch on about 1,200 Islamic terrorist suspects in this country.

THE UNANSWERED QUESTIONS

• Where are the keys to the hire car the bombers left at Luton station?
• Why were unexploded materials left in that car and in the bomb factory in Leeds?
• Was there meant to be a fifth bomber?
• Who was the woman seen by neighbours leaving the bomb factory in the early hours of July 7?
• Did the group get instruction on mixing the explosives, and if so where and when?
• Were the explosives made entirely from household products or did they include military materials?
• Why did the men buy return train tickets if they were intending to die?
• Why did Hasib Hussain, the youngest of the bombers, not accompany the other three on their reconnaissance trip to London?
• Was any weapon found in Jermaine Lindsay’s car?
• Have police traced the identity of all the men who went on a whitewater-rafting trip with Shehzad Tanweer and Mohammad Sidique Khan, including the visitor from Pakistan?
• Where is the rest of the video of Khan’s will and final message? How did al-Qaeda obtain this material? And why didn’t the three others leave a message?
• What have foreign intelligence services told the UK authorities of their information of the four?
• What did the French Interior Minister, Nicolas Sarkozy, mean when he told journalists that “part of this team” had been subject to “partial arrest” in the spring of 2004?

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