In addition, the British authorities are scouring the evidence for clues to whether there is a global dimension to the suspected plot, particularly the extent to which it was planned, financed or supported in Pakistan, and whether there is a connection to remnants of Al Qaeda.
They are still trying to determine who provided the cash for the apartment and the computer equipment and telephones, officials said.
Several of the suspects had traveled to Pakistan within weeks of the arrests, according to an American counterterrorism official.
In 2000, Mr. Rauf's father founded Crescent Relief London, a charity that sent money to victims of last October's earthquake in Pakistan. Several suspects met through their involvement in the charity, a friend of one of them said. Last week, Britain froze the charity's bank accounts and opened an investigation into possible "terrorist abuse of charitable funds." Leaders of the charity have denied the allegations.Several senior British officials said the Pakistanis arrested Rashid Rauf without informing them first. The arrest surprised and frustrated investigators here who had wanted to monitor the suspects longer, primarily to gather more evidence and to determine whether they had identified all the people involved in the suspected plot.
But within hours of Mr. Rauf's arrest on Aug. 9 in Pakistan, British officials heard from intelligence sources that someone connected to him had tried to contact some of the suspects in East London. The message was interpreted by investigators as a possible signal to move forward with the plot, officials said."The plotters received a very short message to 'Go now,' " said Franco Frattini, the European Union security commissioner, who was briefed by the British home secretary, John Reid, in London. "I was convinced by British authorities that this message exists."
A senior British official said the message from Pakistan was not that explicit. But, nonetheless, investigators here had to change their strategy quickly.
"The aim was to keep this operation going for much longer," said a senior British security official who requested anonymity because of confidentiality rules. "It ended much sooner than we had hoped." From then on, the British government was driven by worst-case scenarios based on a minimum-risk strategy.
British investigators worried that word of Mr. Rauf's arrest could push the London suspects to destroy evidence and to disperse, raising the possibility they would not be able to arrest them all. But investigators also could not rule out that there could be an unknown second cell that would try to carry out a similar plan, officials said.Mr. Clarke, as the country's top antiterrorism police official in London with authority over police decisions, ordered the arrests.
But it was left to Mr. Reid, who has been home secretary since May and is a former defense secretary, to decide at emergency meetings of police, national security and transport leaders, what else needed to be done. Mr. Reid and Mr. Clarke declined repeated requests for interviews.Prime Minister Tony Blair was on vacation in Barbados, where he was said to have monitored events in London; Deputy Prime Minister John Prescott did not attend the meeting.
"While the arrests were unfolding, the Home Office raised Britain's terror alert level to "critical," as the police continued their raids of suspects' homes and cars. All liquids were banned from carry-on bags, and some public officials in Britain and the United States said an attack appeared to be imminent. In addition to Mr. Stephenson's remark that the attack would have been "mass murder on an unimaginable scale," Mr. Reid said that attacks were "highly likely" and predicted that the loss of life would have been on an "unprecedented scale."Two weeks later, senior officials here characterized the remarks as unfortunate. As more information was analyzed and the British government decided that the attack was not imminent, Mr. Reid sought to calm the country by backing off from his dire predictions, while defending the decision to raise the alert level to its highest level as a precaution.
In lowering the threat level from critical to severe on Aug. 14, Mr. Reid acknowledged: "Threat level assessments are intelligence-led. It is not a process where scientific precision is possible. They involve judgments."
Reporting for this article was contributed by William J. Broad from New York, Carlotta Gall from Pakistan, David Johnston and Mark Mazzetti from Washington.
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