The foiled plot was "intended to be mass murder on an unimaginable scale," Metropolitan Police Deputy Commissioner Paul Stephenson said.The intelligence that uncovered the plot "makes very strong links to al Qaeda," a senior U.S. administration official told CNN.
The plot involved hiding masked liquid explosives in carry-on luggage, a U.S. government official told CNN.
A U.S. administration official said the terror plot targeted Continental, United, and American Airlines. It was not immediately clear whether other airlines also were involved.
British Home Secretary John Reid said the plotters planned loss of life on "an unprecedented scale."
Britain's threat warning level has been raised to "critical" -- meaning an attack is expected imminently.
The U.S. threat level has been raised to the highest level of "severe," or red, for commercial flights originating in the United Kingdom bound for the United States, according to the Department of Homeland Security.
In addition, the threat level has been raised to "high," or orange for all commercial flights operating in or coming to the United States, the DHS statement said.
The report goes on to say that 21 people have been arrested in the U.K. as part of this plot and that the investigation is ongoing. More to come.