In the early morning quiet, before the sun rose over the Thames, the e-mail alert flashed from New Scotland Yard to overnight newsrooms around London:

"A major terrorist plot to allegedly blow up aircraft in mid flight has been disrupted. ..."

In the dark hours preceding Thursday's terse announcement, British police had rounded up two dozen suspects in the alleged conspiracy. In the tense hours to come, as the sun crossed the Atlantic, the great global hub of Heathrow Airport plunged into turmoil, grim-faced U.S. authorities clamped new controls on millions of travelers, and people on two sides of an ocean once more felt that chilling dread — of faceless terror, of another catastrophe from the skies.

The words from the top were not fully reassuring.

"We cannot assume that the threat has been completely thwarted," U.S. homeland security chief Michael Chertoff told a hastily called breakfast-time news conference in Washington.

Chertoff had just, for the first time in the 5-year-old antiterror campaign, declared a red alert, the highest threat level — in this case for flights from Britain.

Although British and U.S. officials withheld many key details of a continuing investigation, their early disclosures sketched in the outline of a plot to commit what one called "mass murder on an unimaginable scale," wholesale terrorism that one U.S. official said aimed to bring down no fewer than 10 transoceanic jetliners.

For several months, British security had conducted a major covert counterterrorist operation pursuing leads in the case. American agencies were apprised, but it was only in the past two weeks that London's investigators determined that the alleged suicide bombers, in an apparently "accelerating" plot, planned to bring down U.S.-bound planes, Chertoff said.

They were said to be preparing to use liquid explosives disguised as "beverages," to be detonated by harmless-looking electronic devices once they boarded and the planes took off.

As the days ticked down, U.S. officials said, the covert investigators determined that the plotters had targeted specific routes between Britain and U.S. points, and specific U.S. airlines: United, American and Continental.

The sophistication of the devices and the plan was impressive, pointing to a "very determined and very skilled and very capable" group, Chertoff said. And they "were really getting quite close to the execution phase," at the height of the trans-Atlantic tourist season.