WASHINGTON, Aug. 10 — American intelligence officials said today that they and their British counterparts had been tracking terrorists for months before the schemers were rounded up in Britain, and that they could not say positively that all the plotters had yet been caught.
“It was not just a speculative exercise, by any stretch of the imagination,” Deputy Secretary Michael Jackson of the Homeland Security Department said in a telephone interview after two dozen suspects were rounded up in London and its suburbs and in the Midlands of Britain. He called it a “serious, sustained and diabolical plot,” one that was close to being carried out.
In recent days, he said, plotters began investigating nonstop flights from Britain to the United States. An American counter-terrorism official said they planned “a dry run” in the next couple of days and, if they could get on several flights at the same time, planned to carry out the attack within days.
The hijackers of Sept. 11, 2001, also did meticulous research on airline schedules and took numerous rehearsal trips in the United States.
The Homeland Security Department and the Federal Bureau of Investigation sent a memorandum to law enforcement agencies across the country this morning, laying out the thrust of the plot in Britain. The document said the men arrested had been “engaging in advance planning of terrorist attacks against passenger aircraft using liquid-based explosives.”
The memo said officials had no information on a related conspiracy in the United States, “but such a possibility cannot be discounted.”
Another counterterrorism official in Washington offered a similar assessment. “We don’t know whether there is a secondary group or another cell out there,” he said. “We don’t want them to know what we know is out there.”
The F.B.I. and homeland security memo said the plotters expected to use peroxide-based explosives that are “sensitive to heat, shock and friction and can be initiated simply with fire or an electrical charge and can also be used to produce improvised detonators.” Other officials in London said the plotters planned to smuggle the liquids in drink bottles.
Charles Allen, the top intelligence official at the Department of Homeland Security, said he believed that the attacks were not planned around a specific date and were not aimed at any American city.
The F.B.I. and homeland security officials said the peroxide-based explosives the plotters hoped to use to blow airliners out of the sky can be set off with garage-door openers, cellphones, filaments from Christmas tree light bulbs and other seemingly mundane objects that can be converted to killing tools.
“Because of the instability of these substances, spontaneous detonation can occur during the production process,” the memo warned. It noted that peroxide has been used by Palestinian suicide bombers and that Ahmed Ressam, the would-be millennium bomber, had a peroxide-based detonator when he was arrested after entering the United States from Canada in 1999.
“This plot appears to have been well planned and well advanced and in the final stages of preparation,” the joint memo said. “While the operation was centered in Britain, we believe it was international in scope.”
Indeed, the interior minister of Pakistan, Aftab Ahmad Khan Sherpao, said late Wednesday that arrests related to the plot had also been made in Pakistan, and that Pakistani intelligence agencies worked with the British and Americans to head off the attacks. He declined to say how many people had been detained or where they had been arrested.
The American counter-terrorism official who described plans for the dry run said several of the suspected plotters arrested in Britain had traveled to Pakistan in the past two weeks and may have met with at least one person suspected of having links to Al Qaeda.
The suspected Qaeda operative was arrested within the past two days, the official said. That arrest impelled the British officials to round up the two dozen suspects for fear that they would become wary of detection.
All of the plotters are British citizens, most of Pakistani descent, the official said.
A British parliamentary report released in May said that two of the four suicide bombers who carried out the deadly July 2005 attacks on London’s subway system underwent terrorist training in Pakistan and that it was “likely that they had some contact with Al Qaeda figures” in Pakistan.
Although some details remain unclear, the terror alerts generated by the arrests in Britain had a far grimmer tone than some of those previously issued by the Department of Homeland Security, whose use of color-coded warnings in the absence of specific information gave rise to apathy and derisive jokes.
While the focus was on Britain today, officials in Washington said American intelligence agencies had been deeply involved in the investigation. One question sure to be asked as more details emerge is whether the arrests indicate that the post-Sept. 11 American intelligence bureaucracy has passed its first big test.
The Department of Homeland Security was created after the 2001 attacks to unite many agencies that had previously been far-flung. And a new post was created, that of national intelligence director, now occupied by John D. Negroponte, to stamp out the inter-agency bickering that was blamed for much of the failure to detect and dismantle the Sept. 11 plot.
For now, President Bush was hailing the arrests as a wonderful example of cross-ocean cooperation with Britain and the government of Prime Minister Tony Blair, his strongest ally in the war in Iraq. “The cooperation on this venture was excellent,” Mr. Bush said.
One question arising from the sudden new airport-security measures put in place today is how long they will last, or whether they might become permanent, as long lines and long waits at check-in counters and shoe inspections have become routine in the post 9/11 world of flying.
It seemed clear that the arrests in Britain could reverberate in domestic politics in the United States.
President Bush said the arrests were yet another reminder that the United States “is at war with Islamic fascists,” and that while the United States is safer than it was before Sept. 11, 2001, dangers remain.
“It is a mistake to believe there is no threat to the United States of America,” Mr. Bush said in Green Bay, Wis., before heading to a Congressional campaign event. “And that is why we have given our officials the tools they need to protect our people.”
The president was almost surely alluding to his administration’s heightened surveillance of telephone calls, electronic communications and international money transfers — measures that his critics have called high-handed infringements on civil liberties, but that he has described over and over as essential weapons in a continuing war on terrorism.