If a Bomb Brought Down EgyptAir 804, the War on Terror Is About to Change

First, the caveats: No hard evidence of a bomb has been found, and no claim of responsibility has been announced. But because of the way EgyptAir Flight 804 dropped out of the sky, and the fact that it was headed to Egypt—scene of the only airliner bombing in 14 years—government officials and outside experts agree that the odds favor a terror strike.

No one yet knows how it might have happened, but two main possibilities present themselves—and both present mind-boggling implications.

Read More: Airport Workers in the Spotlight After EgyptAir Crash

The first and perhaps most likely possibility is that the airliner was brought down as airliners usually are—by a bomb sneaked onto the plane at its last point of departure. That was Charles de Gaulle Airport (CDG) in Paris, which was thought to be among the most secure in Europe. If the bomb got onto a plane at CDG, it calls into question all the preventative security measures taken to safeguard global civil aviation since 9/11, because no city would be taking safety more seriously than Paris since the attacks of last November, to say nothing of the Brussels bombings of March.

If Paris was where a bomb got on board a plane, the spotlight then focuses on the 86,000 airport workers cleared to work in the so-called “reserved zone” beyond the reach of passengers. That’s a huge number of people to vet for security; and in France, where 7 percent of the population is Muslim, the fear is that any of them could have quietly gone to work for ISIS. As TIME’s Vivienne Walt reports, French security services already have tried to detect evidence of radicalism among airport workers, and in fact 57 workers lost their clearances through November of last year. In Belgium, something similar happened at nuclear power plant, where 11 workers were barred after concerns were raised about “insiders” stealing radioactive material for use by ISIS.

This is part of what makes ISIS so challenging. The group does not operate like al-Qaeda, which saw itself as an elite, members-only strike force that put immense thought and planning into choosing its targets. ISIS operates more like a mass movement with a very low bar to entry. The world’s perhaps 1.2 billion Sunni Muslims are its declared constituency, but anyone can join—”anyone who is angry, really,” as former ISIS hostage Nicolas Henin told me recently—and all are encouraged to stir up mayhem where they can. A Russian jet carrying 224 people was brought down last November over Egypt by a homemade bomb tucked into a 12-oz. aluminum beverage can and rolled onto the charter flight with the other catering.

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The assumption is that the downing was the work of ISIS’s affiliate in Egypt’s Sinai Peninsula, which took credit for the attack. But it could have been anyone. ISIS can be anywhere there’s an IP address and a mind open to its message. That’s the first of the mind-numbing possibilities of EgyptAir 804.

The other possibility is that the bomb was put on the Airbus 320 before it reached Paris. That’s not how terror strikes usually work against airplanes, but it’s been done. Al Qaeda’s deadly Yemen affiliate in 2010 secreted homemade explosives in printer cartridges, then shipped to the United States. They were found, thanks to a tip from Saudi intelligence, in the hold of cargo jets during scheduled stops in Britain and Dubai.

Read More: Here’s What We Know So Far About EgyptAir Flight 804

The plane that became EgyptAir 804 the moment it left Paris had, in the previous 48 hours, been to Asmara, Eritrea, and Tunis, Tunisia. Eritrea is a police state, though in the convoluted regional politics of Africa’s Horn it has a history of supporting Islamists in nearby Somalia against its arch-enemy, Ethiopia. But it’s Tunis that draws the eye. Leaders of the north African nation won the Nobel Peace Prize for forming a government that sustained the promise of the Arab Spring. But their country is also a recruiting hotbed for ISIS , providing more volunteers than any other nation to the war in Syria. It’s not hard to imagine there might be one or two workers thinking of ISIS at Tunis-Carthage International Airport.

Or, for that matter, at the airports where the same plane made stops in the previous days: Khartoum, Riyadh, Medina, Juba. And why rule out Cairo and Alexandria, the largest cities in an Egypt, where the government of President Abdel Fattah el-Sisi has equated all expressions of political Islam with terror, forcing millions to take sides? Airplanes are supposed to be searched at every stop, but it’s the job of the airline itself to conduct the search, and anyone who has passed through an Egyptian airport knows how what cursory looks like. And if airline employees are being doubted, wouldn’t that include a mechanic, who could know where to tuck something out of sight? Entirely possible.

But here’s the thing: There are 100,000 flights every day around the world. Airport security monitors the safety of every one. If that task is expanded to take in the recent itinerary of every plane—with a leery eye cast toward stops in the Middle East—the mind boggles again, and goes right on boggling. We quickly approach the level of paranoia that is the ultimate aim of any terror attack, and, with that paranoia, a creeping suspicion toward all Muslims that ISIS explicitly says it wants to encourage. And what’s the good in that? Everyone wants to feel safe. And the best way remains to look out for one another, and not only in the security sense of the expression.