'Plot Would Have Killed Thousands'

"I got a call telling me that it looked as if the focus had turned on an attack on the United States, specifically an attack on airliners leaving from Britain, traveling to American cities," Chertoff said. "It also became evident, within 24 hours, that the time frame within which the attack was going to take place, would not be a matter of months but … a matter of weeks or even days."

Airports in the United States and the United Kingdom were put on red alert — meaning a potential attack could be imminent — and liquids were banned from carry-on luggage as suspects were picked up, including 24 British-born Muslims and seven Pakistanis.

"We had to start about 9, 10 o'clock in the evening, when the arrests began to go down in Pakistan, and when we were first given the ability to tell other people about the plot," Chertoff said. "And we had to turn the entire process around by 6 a.m. the following morning, before people started to board airplanes.

"You had to change literally thousands of people's behavior in the course of about 12 hours. We had to train them. We had to get everybody to understand what the new rules were going to be. And you had to communicate to the public in a very short period of time.

"And so, we spent literally the entire night bringing in not only the TSA senior leadership, but also talking on the phone to the airline leadership, so that everybody would understand what needed to happen at 6 a.m. the following day," he said.

For Chertoff, the concern remained that an attack would have been carried out if they'd missed a critical detail. "There's an enormous sense of working against time, giving the analysts as much time as you possibly can, but always recognizing at the end that the benefit of the doubt has to be in favor of saving lives."

Assessing Current Risks

Since last August, the failed plot has had an enormous impact on U.S. airports, which have remained on orange — or high — alert, for nearly a year.

After authorities tested the explosive liquids, the government determined what quantity of liquid explosives could pose a risk if smuggled onboard flights, leading to the 3-ounce limit for carry-on bags.

Passengers are still restricted when bringing liquids onboard, and those rules may remain in place forever.

At the moment, Chertoff believes there is a "heightened risk" of an attack.

"We have seen that in some areas of Pakistan, the enemy has been able to reconstitute itself and get a breathing space, so to speak, where they can plan and do some recruiting and some training. We've seen increased effort to develop terrorist operatives in Europe.

"And, of course, the concern we have, because of the visa waiver program, has been Europeans either carrying out attacks against Americans on the European continent, or even coming to the United States," Chertoff said.

"When you add these things together, they don't move into a mathematical certainty we're going to have an attack, but they do suggest that there is a heightened threat, a bit more capability than there was, and, therefore, all the more reason for us to continue to raise the level of our security and our defenses," he said.