U.S. and British officials say this group was under such close surveillance that the police virtually held the on/off switch, able to shut it down at will.
Had it actually happened, it would have involved, at its core, two commonly available chemicals brought through security individually, then mixed up on board. Experts have little doubt that if they did it right, it would have worked — to potentially devastating effect.
"It could be very dangerous on a plane," says chemical expert Neal Langerman. "If they mixed it, it would ignite and most likely detonate instantly on mixing."
Explosives experts say it could have caused a severe fire on board or, even worse, blown a big enough hole to bring down a jumbo jet. But it never got that far.
British investigators were monitoring Internet cafes the terror suspects used, keeping tabs on the flow of their money, and watching their travel and phone calls.
Such tight control, U.S. officials say, that after months of intense surveillance there was almost no chance any of the plotters could have actually carried out their attacks.
"In this particular case, the British government made the calculation [that]now was the appropriate time to take action," says U.S. Attorney General Alberto Gonzales.
Both U.S. and U.K. officials say it was a tip from a member of the Muslim community that helped focus police there on what turned out to be the cell.
Since, 9/11, US officials have aggressively courted the Muslim community here, knowing just that kind of tip can be essential.
Just today, Joseph Persichini, the head of the FBI's Washington office, met with Muslim leaders to encourage such cooperation.
"That power, coupled with all of us together, will strengthen this nation as best as we possibly can," says Persichini.
But while investigators believe they've arrested or identified all involved, they cannot be certain — one reason airline security here remains tight.