Homeland Security Secretary Michael Chertoff said there was no indication of plotting in the United States but said officials cannot assume that the terror operation in Britain had been completely thwarted.
LONDON (AP) - British police said Thursday they had arrested 21 people in connection with a terror plot against airlines traveling from Britain to the U.S. which was an "attempt to commit mass murder on an unimaginable scale."
The terrorists were apparently targeting Continental, American and United flights.
London's Heathrow airport, the busiest airport in Europe, was closed to most European flights Thursday morning after officials raised Britain's national security alert to its highest level - suggesting a terrorist attack may imminent. Huge crowds formed at security barriers.
Police carried out the arrests overnight in London, its suburbs and Birmingham as part of a major covert counterterrorism operation that had lasted several months, Deputy Commissioner Paul Stephenson said. The identities of those arrested were not immediately released.
Police said searches were continuing in a number of locations. "We have been very successful in arresting those we were targeting but this is a lengthy operation, and no doubt there will be further developments," Stephenson told reporters outside the headquarters of the Metropolitan police. "We think this was an extraordinarily serious plot and we are confident that we've prevented an attempt to commit mass murder on an unimaginable scale," he said. He declined to say how many aircraft may have been targeted, or which airports might have been involved. He also refused to say whether the suspects were British citizens or foreigners.
Prime Minister Tony Blair, vacationing in the Caribbean, had briefed President Bush on the situation overnight, Blair's office said. There was no immediate public reaction from the White House. Bush is spending a few days at his ranch near Crawford, Texas.
Britain's Home Secretary John Reid said the alleged plot was "significant" and that terrorists aimed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."
The U.S. government responded to the announcement by raising its threat alert to its highest level for commercial flights from Britain to the United States amid fears the plot had not been completely crushed.
"We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted," said U.S. Homeland Security Secretary Michael Chertoff.
It is the first time the red alert level in the Homeland Security warning system has been invoked, although there have been brief periods in the past when the orange level was applied.Homeland Security defines the red alert as designating a "severe risk of terrorist attacks."
Chertoff added, however, there was no indication of current plots within the U.S. No major U.S. carriers immediately announced they were canceling flights to Heathrow.
A senior U.S. counterterrorism official said authorities believe dozens of people - possibly as many as 50 - were involved in the plot, which "had a footprint to al-Qaida back to it." The official spoke on condition of anonymity because of the sensitivity of the situation.
The plan involved airline passengers hiding masked explosives in carry-on luggage, the official said. "They were not yet sitting on an airplane," but were very close to traveling, the official said, calling the plot "the real deal."
Passengers in Britain faced delays as tighter security was hastily enforced at the country's airports and additional measures were put in place for all flights. Laptop computers, mobile phones, iPods, and remote controls were among the items banned from being carried on board.