The world learned of a terrorist plot Thursday that would have caused mass death and destruction aboard a number of passenger jets had British authorities not aggressively investigated and arrested many of the plotters.
More than 20 suspected terrorists were arrested in England by early Thursday morning, in an operation that involved British intelligence, Scotland Yard and assistance by a number of other law enforcement and intelligence agencies, including those in Pakistan.
ABC News has learned that two "significant arrests" in Pakistan in recent days may have significantly accelerated the pace of the investigation.
Many of the alleged terror plotters appeared to be of Pakistani descent. It appears that they were probably "homegrown" terrorists with strong links to al Qaeda and Pakistani operatives. This new generation of terrorists have figured significantly in plots in the U.S., London and Canada in recent months.
In this case, the plotters apparently intended to assemble small but powerful bombs in flight and use them to take down flights from England to the United States.
Airport security was tight in both nations. A "red alert" -- the highest alert level -- was issued in the U.S., and a "critical" state was issued in England. Passengers are undergoing intense scrutiny -- carry-on baggage of almost all kinds has been eliminated in Britain and delays abound at London's Heathrow, the world's busiest airport.
According to a Department of Homeland Security briefing to the aviation sector, the terrorists appear to have planned to use multiple persons aboard each flight to assemble peroxide-based liquid or gel high explosives. The bomb-making materials could easily be concealed in small containers -- water bottles, tooth paste tubes, juice boxes and any of the other numerous person items passengers traditionally take into the passenger compartment of commercial flights.
At least nine transcontinental flights from American, United and Continental airlines were targeted in the plot. ABC News has learned that terrorists planned to attack the planes three at a time, waiting an hour between each attack.
According to federal authorities, two or three bombers would each carry a separate portion of the bomb onto the plane to avoid detection. Once onboard the bomb would be assembled and then detonated by using heat or friction.
British authorities had been tracking some of the suspects for several weeks but stepped in to round up the plotters when they began to book flight reservations.British authorities have shared parts of the investigation with the FBI, and out of concern for leaks, only the barest details were shared with regional authorities as late as last night.
Now there is a continued concern that other members of the cell remain on the loose and may remain a present danger to intercontinental air traffic as well as air traffic in Europe. Raids were expected to continue in England throughout the day, and authorities were said to be seeking the "factories" where the bomb parts were prepared.
U.S. authorities, meanwhile, were running down leads to ensure no plotters or associates were within U.S. borders and intent on causing harm.