British authorities said Thursday they had thwarted a terrorist plot to simultaneously blow up several aircraft heading to the U.S. using explosives smuggled in carry-on luggage, averting what police described as "mass murder on an unimaginable scale." Home Secretary John Reid said British police and government were confident the main suspects had been arrested.
Huge crowds formed at security barriers at London's Heathrow airport as officials searching for explosives barred nearly every form of liquid outside of baby formula. [...]
The U.S. government responded 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. The alert for all flights coming or going from the United States was also raised slightly.
In Washington, two U.S. counterterrorism officials said the terrorists had targeted United, American and Continental airlines. They spoke on condition of anonymity because of the sensitivity of the case.
Police arrested 21 people in London, its suburbs and in Birmingham as part of a major covert counterterrorism operation that had lasted several months, Deputy Commissioner Paul Stephenson said. Searches continued in a number of locations.