British police say they have arrested 21 people in connection with a terrorist plot to blow up aircraft flying from the United Kingdom to the United States.

The plot was "intended to be mass murder on an unimaginable scale," Metropolitan Police Deputy Commissioner Paul Stephenson said. The UK's threat warning level has been raised to "critical" -- meaning "an attack is expected imminently."

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The plot involved hiding liquid explosives in carry-on luggage, and some flights involved in the plot would have been going from the UK to New York, Washington, D.C., and California, U.S. government officials said.

The U.S. has set the alert level to Red for all incoming flights for the first time. All other flights have been upgraded to Orange. It is crystal clear that these people don't intend to stop trying to attack us and I have no doubt that things would be worse if we weren't there. Their goal is to make the world an extremist Islamic nation and anything short of that is unacceptable. Anyone who thinks they just want us out really hasn't been following the war at all.

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