LONDON — A team of British intelligence analysts has drawn up a chilling scenario in which terrorists launch a swarm of small drones in an attack on a major sporting event like the Super Bowl, unleashing multiple explosive devices on the crowd in the stadium.

“If we do not act to prevent it, it is only a matter of time,” Chris Abbott, the executive director of a think tank called Open Briefing, told The Daily Beast in an exclusive interview.

Abbott’s group, which calls itself “the first civil society intelligence agency,” includes former military specialists and intelligence agency operatives. They have been tracking the development of drones for several years.

What they now see is a cheap and easily accessible technology that is particularly suited to the limited resources and ability of the small, widely dispersed terrorist sleeper cells that are known to exist in Western Europe and the United States—or lone wolves indoctrinated by ISIS or al Qaeda, like the San Bernardino attackers.

The experts believe that ISIS has already recognized the opportunity provided by off-the-shelf drone technology in its planning of attacks on Western cities.

“ISIS are already using drones in Iraq and Syria for intelligence gathering, quite successfully, for battlefield awareness,” says Abbott.

“They are in direct competition now with al Qaeda and are desperate to launch a mass casualty attack on Western targets. A swarm drone attack against a large sporting event would be potentially disastrous,” he said. (In the Paris mass attacks a major stadium where the French president was watching a soccer game was targeted but a suicide bomber was deterred by the gate security and blew himself up outside.)

“ISIS have demonstrated themselves to be very sophisticated in terms of propaganda. The drones would be equipped with cameras so that they could film the actual attack as it takes place and live stream it on the Internet.

“These things would have an effect long beyond the horror of the actual event, a very visual impact.”

Abbott points out that the so-called Islamic State widely known as ISIS has distributed videos showing the training of those who carried out the terrorist attacks in Paris and that drone attacks would probably be similarly planned in Syria as cell leaders are trained in the acquisition and use of drones.

In January Abbott’s group published a report, through the London-based Remote Control Project, that analyzed the considerable arsenal of small drones now available and discussed their potential for terrorists. That report has prompted British lawmakers to urgently review what steps should be taken to anticipate such attacks and prepare defenses against them.

“The tricky thing,” said Abbott, “is that there is no individual counter-measure that could be effective. You have to have a layered defense that starts with the regulation and registration of drone ownership, has a second layer that restricts the ownership of larger and more capable drones, then you need systems that can warn of attacks and possibly jam the channels being used to control the drones and, as a last line of defense, the ability to intercept the drones and shoot them down.

“Unfortunately, if you have to resort to shooting them out of the sky in an urban environment or at a packed stadium, the risk of collateral damage is very high.”

At the same time Abbott acknowledges that regulators face a dilemma: “There is so much legitimate use for drones, they serve a very useful purpose, including in search and rescue, that you don’t want to kill the innovation, you don’t want so many restrictions that it becomes impossible to use them.”

In the U.S. (where 700,000 drones were sold last year alone), the Federal Aviation Administration now requires that recreational drone users register their personal details when they buy a drone, but regulations covering the use of commercial drones weighing more than 55 pounds have been delayed for more consultations with industry lobbyists.

In Britain, lawmakers are calling for what is known as “geo fencing”—in which sensitive sites like airports, official buildings, ports, nuclear power stations, and electricity power grids are designated as no-fly zones and all drones are fitted with firmware that automatically locks out the drone from entering the zones.

Abbott admits, though, that such firmware can easily be disabled. “Our best defense, as always,” he says, “is solid intelligence that can prevent an attack before it is attempted.”

Commercial aviation remains a prime target for terrorist attacks. But with airports in Western Europe and North America turned into fortresses, soft targets in aviation are far harder to find. Consequently Abbott fears that drones could present yet another example that our defenses against terrorism remain prioritized against past threats and dated technology rather than being realigned to anticipate a completely new threat before it can become lethal.

And he stressed just how lethal even a small drone could be.

“A consumer hobbyist drone can carry the equivalent of a pipe bomb, with the equivalent of five to 10 kilograms of TNT, or of a suicide vest, of roughly between four to 10 kilograms of TNT, or of the improvised explosive devices, IEDs, used for years in Iraq and Afghanistan. A mass drone attack would be the equivalent of multiple suicide bombers being launched at a single target at the same time.”

On the other hand, a single drone directed at a single, high value target—like a political leader—could also have devastating consequences. And there are precursors that expose our vulnerability to such attacks:

In 2013 a small camera-carrying drone landed directly in front of German Chancellor Angela Merkel when she was addressing a rally in Dresden. The drone was operated by the German Pirate Party, as part of a protest about government surveillance techniques. Last April a small drone carrying a sample of radioactive sand from the Fukishima nuclear plant landed on the roof of the Japanese prime minister’s offices in Tokyo.

Had each of these drones been weaponized, their feats would not have faded so readily from the headlines.

Abbott explains that carefully prepared and targeted attacks like these would be difficult to intercept. “All we are able to do at the moment is to put increasingly sophisticated hurdles in their path so that they become increasingly less likely to succeed.”

As well as attempts to take out political leaders, relatively small drones are capable of carrying other payloads of mass terror, like chemical weapons and poisons that could, for example, contaminate water supplies. Abbott also points out that a drone could deliver a radioactive “dirty bomb.”

“The actual destructive effect would be limited, but the psychological and economic effects could be substantial.”

But even if a defense system is able to disable the GPS guidance system of a drone, a terrorist can now have the means to send a drone from afar like a cruise missile on a pre-determined and programmed path to the target.

“You can get systems quite cheaply now that use inertial navigation sensors, like a plane, pre-programmed with way points and using dead reckoning for the distance and timing—an example of capabilities that you wouldn’t even have imagined five years ago.”

Indeed, I recalled the 1991 attempt by the Irish Republican Army to strike 10 Downing Street while the British War Cabinet, led by the Prime Minister John Major, was meeting. From a van on a nearby street the IRA launched several mortars and one landed in the garden of Downing Street, only yards from the meeting. No politicians were injured but four others, including police officers, were hurt.

Abbott agreed that today’s drone technology would assure that, if undetected, such a strike on a head of state and his cabinet would now be far more accurate and deadly.

In effect, what began as a high-cost and exclusively military technology has been consumerized. And so, ironically, as we continue to use military drones for the targeted assassinations of terrorists, they are now in a position to turn the same technology against us—albeit in an improvised and less systematic form, but no less threatening for that.

To successfully counter drone attacks, Abbott summarizes the necessary steps as “foil or fail.”

He says that ISIS would need to train cell leaders to a high level of proficiency in handling and launching drones and that the best chance of foiling an attack would be when the intelligence agencies pick up the movement of such individuals during the course of acquiring drones and preparing and practicing for an attack.

“At this point that’s our best hope. The second best hope is that if the attack goes ahead it fails, due to the limitations of the drone or the pilot or because our active or passive defenses bring down the flight.”

And if not…

There is the specter conjured by Thomas Harris’s 1975 novel Black Sunday (and the movie), where an attack carried out by the Palestinian Black September group, assisted by a demented technician, flies a Goodyear blimp toward Tulane Stadium in New Orleans during the Super Bowl, loaded with a quarter of a million steel darts that will be fired to eviscerate the spectators.