If a terrorist wanted to find the most vulnerable point in America’s airport network they could not have hoped for a better guide than what just happened at Atlanta Hartsfield-Jackson.

Just after 1 p.m. Sunday the whole airport, the world’s busiest, went dark for 11 hours. Thousands of flights were disrupted. For many hours nobody in authority attempted to explain—or even seemed able to explain—what had happened.

Just imagine this is a classic plan for phase one of a terrorist attack: Render the target blind. None of the defenses are operational. Thousands of people are trapped in restricted space without directions about how they can find an exit.

As chaos spreads nobody knows who turn to for information. The communications blackout is as complete as the power blackout.

Given this situation a small band of suicide bombers could roam freely and commit mayhem and massacre on an unprecedented scale.

Initial responses as the story broke were that the holiday season travel would be disrupted for days. That is true. The ripple effect of the paralyzed airport will be worldwide as thousands of international connections are canceled.

“ Just imagine this is a classic plan for phase one of a terrorist attack: Render the target blind. ”

In the United States it will probably take a week to get schedules back in service. As scores of airplanes sat on the runway or at gates frozen for lack of power the aircrews ran out of their allowable time on duty. As of Monday, more than 1,200 flights were canceled as a result of the Atlanta blackout.

But forget about the harm done to Christmas travel. It’s much more serious than that. There has never been a single-point failure of this magnitude in any major airport in the U.S. All the essential systems seem to have lacked backup—or, in the language of the bureaucrats, redundancy.

Normally around 275,000 passengers, equal to the population of a small city, pass through Atlanta’s airport every day. From the moment when this disaster hit that flow continued without restraint, with the terminals quickly becoming jammed. Nobody in the airport management stepped up to stop that happening.

This is not the first time that this airport has been hit by a power failure that caused chaos. In August 2016 the operations center for Delta Air Lines, for whom Atlanta is its major hub, lost power causing the airline’s computer system to crash. More than 2,000 flights were canceled over several days.

There was another failure in January this year when all Delta’s domestic flights were again affected by a computer crash and 300 flights were canceled.

Immediately after the August 2016 systems failure Delta blamed the local utility, Georgia Power, for the problem, but later retracted that and admitted that its own IT systems were to blame, as they were again in January. In the case of both failures Delta did not explain the lack of backup systems.

This time the problem does appear to have originated with Georgia Power. They said a fire in an underground plant providing power to the substations at the airport caused “extensive damage” to the facility. Such outages, they said, “were very rare.”

But of course this does not explain the glaring problem that has surprised and shocked national security experts: Why could a failure at one power source automatically knock out the supply to a whole airport? Why were there no backup systems to keep the essential services at the airport functioning? Why were there no emergency generators ready to cut in as they are, for example, at hospitals? Why was there no power for the most basic systems, not even lighting for the terminals, leaving passengers and airport staff in the dark at gates and security checkpoints?

As power was restored at midnight Kasim Reed, Atlanta’s mayor, said the fire was so fierce that it had engulfed the switch that was supposed to automatically trigger backup power—suggesting that the design of the system’s redundancy was badly flawed.

The Atlanta chaos is yet another red flag indicating that our airports are far from ready to deal with a terrorist threat.

In January there were 12 hours of chaos at Fort Lauderdale, Florida airport following a shooting that killed five people in the baggage claim area. Terrified passengers fled terminals without direction, even on to runways; security staff abandoned their posts; panicking police officers jammed the emergency radio system, and SWAT teams appeared but did not know where to land their helicopters.

In August 2016 a false report of gunfire at a security checkpoint at New York’s JFK Airport led to similarly widespread panic and lack of any coherent emergency response.

In the first presidential debate of the 2016 campaign Donald Trump said—correctly—that flying into U.S. airports often felt like flying into a third-world country. He vowed to spend $500 billion on infrastructure, including our airports. What ever happened to that promise?

The Atlanta fiasco exposes both the emptiness of that pledge and the extremely dangerous vulnerability of the world’s most heavily traveled airport network.