Imagine looking out the large plate-glass windows at London’s Heathrow Airport onto the tarmac, waiting for your flight. As you idly watch these wide-body planes, you might notice they’re stacked up on the runway, waiting to take off. You might count the planes and see seven in line, all bound for North America. If they took to the air on schedule, they’d be somewhere over the Atlantic Ocean or an American city around the same time.

Now imagine, a few hours into their flight, all seven planes exploded, like clockwork.

This was a very real conspiracy in the post-9/11 era, and the terrorists—several hardheaded British nationals trained by al-Qaeda’s senior bombmakers—might have pulled it off, had it not been for a massive Anglo-American-Pakistani effort to stymie it. The unsettling thing, as I discuss in my forthcoming book, Disruption: Inside the Largest Counterterrorism Investigation in History, there’s little reason to believe it shouldn’t—couldn’t—happen again.

Here’s an even more head-scratching outcome that suggests larger, hidden gears were spinning, ones I didn’t quite realize were in motion until I finished writing two-thirds of the book. After the I.S.I., Pakistan’s fearsome intelligence service, captured the conspiracy’s mastermind at an otherwise unremarkable Pakistani railway intersection, it decided to keep the prisoner from speaking to the outside world—including to the Americans and the British.

The Silent Treatment

This operative, a Birmingham, U.K., native named Rashid Rauf, was of particular interest, since he had also quarterbacked the most lethal terrorist attack in London’s history: the “7/7” operation, where four men with explosive-laden backpacks simultaneously struck the Underground and a double-decker bus at morning rush hour on July 7, 2005, killing 52 people and wounding hundreds.

The terrorists behind a plot to blow up seven planes leaving from London Heathrow almost succeeded. Only a massive Anglo-American-Pakistani effort managed to stymie it.

Rauf had also trained another terrorist cell in London that tried to carry out a similar attack two weeks later. Luckily, they failed because they bungled when mixing the chemicals needed to create the explosive. Building bombs is harder than it looks.

So Rauf had a hand in at least three terror operations in the West, with the final one holding the potential to kill hundreds, if not thousands, of people over the Atlantic. This last plot involved smuggling liquid explosives onto planes in the plastic bottles of sports drinks. (This is the reason why we have the “3-1-1” rule for liquids on aircraft.) Rauf’s capture at the railway junction jump-started a global dragnet to crush his global operation before word leaked out that he had been arrested. Who knows what else he had planned?

Yet Rauf became the only incarcerated al-Qaeda operative—or perhaps one of just a handful—in the two decades since 9/11 to be silenced.

This was during the era when Pakistan routinely turned over terrorists to the Americans, including many currently incarcerated in the detention facility at Guantánamo Bay, Cuba. I interviewed most of the C.I.A.’s now retired senior leaders from that period, and when I asked whether there were other al-Qaeda operatives in the past 20 years that Pakistan had refused to let the U.S. interrogate, the answers were something to the effect of “Hmm, now that you mention it, none come to mind.”

The kicker? In December 2007, Rauf mysteriously escaped from police custody following a routine court appearance in Pakistan’s capital. To this day, we don’t definitively know who helped him disappear, where he went, or whether he’s alive or dead. Following his escape, he had a hand in a few other foiled terror plots, including one targeting New York City subways, and then went silent. He’s probably dead, but al-Qaeda never formally acknowledged his “martyrdom.”

Which brings us back to why Pakistan never allowed the U.S. or U.K. to interrogate Rashid Rauf in the first place, despite repeated requests. It turns out that when Rauf started talking to his Pakistani interrogators, he insisted he was a “Kashmiri freedom fighter,” a soldier in the decades-long Pakistani effort to subvert Indian rule in the disputed region. Moreover, he claimed he had trained in Kashmir, which meant he was connected to Pakistan’s spy agency. He was also the brother-in-law of one of the most prominent jihadists in Pakistan, who had worked with the I.S.I. for years.

Rauf was fruit from the poisonous tree. Sure, his claim to fame was blowing up targets and killing civilians in the West. But he first started out on a mission approved by the Pakistani government. If this information were to become public—as it would once American interrogators started asking questions—it would be embarrassing for the I.S.I. The Pakistanis needed to keep this information under wraps.

Thus, the I.S.I. devised a simple solution: their prize prisoner would simply become permanently unavailable to chat. And when he escaped, with or without official assistance, everyone figured the C.I.A. would eventually track him down and launch a Hellfire missile in his direction. If that happened, the Americans would have killed a terrorist who had tried to murder thousands of civilians, and Pakistan’s dirty secrets would die with him.

Everyone wins.

Aki J. Peritz’s Disruption: Inside the Largest Counterterrorism Investigation in History will be published on December 1 by Potomac Books