Secret missile launchers were uncovered. Fighter jets scrambled into the air. The White House went into lockdown. This was not a drill.
Last week, the Battle of Washington DC was just a hair-trigger away.
It was 8.27am on November 26 when air-defence computers picked up something unexpected.
It was an unidentified target. It was a "slow-moving blob" relentlessly flying towards the heart of the US capital.
Whatever it was, it wasn't supposed to be doing that.
Washington DC is surrounded by a rigorously enforced "no-fly zone". The events of September 11, 2001, are forever in that nation's mind. So, an "Aircon Yellow" alert was issued.
Warning calls went out by radio. Attempts were made to identify the aircraft.
There was no response.
The target continued to get closer. Soon, it crossed another no-go line.
Just minutes from potential disaster, emergency protocols swung into effect.
"Aircon Orange" was declared.
Security officials ran to their offices. Washington's defenders dutifully followed their training. The White House and Capitol Building were put on alert.
"They have strict protocols for how they handle these situations," retired US Air Force Colonel Cedric Leighton told CNN.
• 250m rule catches 'White House' brothel
• White House says it will not take part in impeachment hearing
• The White House: Melania Trump unveils extravagant Christmas decorations
• 'Walkout' revelation: White House officials considered 'midnight self-massacre', according to explosive book
WHITE HOUSE, RED ALERT
Phone lines buzzed between civilian and military agencies. The Federal Aviation Administration said it had no untoward aircraft on its radar.
Something was wrong.
Billions had been spent on tracking and identifying everything in the air for 50km around the White House and Capitol Hill. Absolutely nothing except approved military and emergency services aircraft is allowed within 25km.
There were powerful civilian and military radars. But there was also a carefully positioned network of infra-red and electro-optical cameras, sound sensors and other secret devices – all intended to give early warning of a potential attack. They were designed to detect threats ranging from a kamikaze Cessna light aircraft through to cruise missiles.
All are especially sensitive: They're supposed to see through stealth and find what more conventional air traffic systems cannot.
This particular morning, none were producing clear results.
And time was pressing.
A light aircraft could reach the White House after entering the no-fly-zone in just 15 minutes. A business jet would take only five.
Even though this was a slow-moving target, those tasked with defending the heart of US politics had just seconds in which to make life-and-death decisions.
Little wonder the panic button was pressed.
Confusion reigned. But there was no time for hesitation.
At the nearby Andrews Air Force Base, a flight of F-16 Viper fighter jets were rolled out to the runway – engines and weapons hot. A US Coast Guard MH-65 Dolphin helicopter with the call sign "Blackjack" clawed its way into the air from Reagan National Airport in an attempt to identify and ward off the intruder.
This was the most visible element.
But, behind the scenes, activity was frantic and widespread.
Air defence is a layered, concentric system.
Aircraft provide the outermost ring of interception.
But behind them are overlapping circles of medium-range surface-to-air missile defences.
In the case of Washington DC, this includes the Norwegian-built National Advanced Surface to Air Missile System (NASAMS).
These launchers are scattered throughout Washington's suburbs. They sit in plain sight – usually quietly – among parks, off significant roads or in government facilities. Each consists of a compartmentalised box holding six missiles sitting on a semipermanent platform.
Scattered around them are radars, optical telescopes and command facilities. Sometimes these are on mobile trailers. Sometimes they are part of buildings and other infrastructure.
Then there is another, last line of defence.
Sitting amid Washington's most sensitive sites are Avenger short-range point-defence missile systems. They're built to take on anything that gets past the fighter jets and medium-range missiles as well as drones that may appear from among the suburbs.
So, when a secret surface-to-air missile system suddenly pops up on a building across the street from the White House, it's a sign things are getting serious.
LAST LINE OF DEFENCE
CBS reporter Sara Cook watched in alarm as a menacing machine appeared on the roof of a nearby office building. It had eight rectangular pods and a .50 calibre machine gun.
It was an Avenger air defence system.
Usually carried by army Humvee trucks, this one was on a retractable pedestal.
The pods contain Stinger ground-to-air missiles. With a range of about 3km, they are agile and fast – though they carry a relatively light infra-red guided warhead.
These were armed and ready.
With the entirety of the surrounding Capitol Hill in lockdown, Cook could only watch – and wait.
Overhead, the Coast Guard helicopter Blackjack had reported it saw nothing suspicious in Washington's skies, though it did note it had almost flown into a flock of birds … but this was not enough for a stand-down to be issued.
"There's quite a bit of talk about some rogue states as well as terror groups using swarms of drones to conduct attacks. Such attacks could show up on radar as a 'blob', and that would make it really difficult to get a positive ID on such a radar target," Col. Leighton said. "So, it's definitely possible that a future situation in which a radar 'blob' … could mask an attack."
Which is why Washington's Mall was locked down and the covers taken off concealed Avenger missile systems.
The fallout was widespread, especially among staffers on Capitol Hill.
"Officers had no information, and there was zero communication to members of the Capitol Hill community as the situation was unfolding," one public servant complained.
Some 45 minutes after the initial alarm, an all-clear rang out.
WHAT WENT WRONG
What was this unidentified flying object over Washington DC? Was it a flock of birds? A balloon? A spy plane? A cloud of pollution?
CNN reported one source as saying that's "still not known … a big slow-moving blob of something. I bet it was a big flock of birds seen on radar".
But, according to the blog The Warzone, defence sources have revealed the alert was the result of compounded errors.
A telescopic anomaly combined with human error to blow a computer glitch out of all proportion.
Apparently, a perfectly valid aircraft flying under all the airspace rules and with its identification transponder turned on produced an unexpected "echo" within the air detection network.
It is called a "mirror track", where the networked computer system incorrectly invents a separate aircraft in a different location.
Automated verification systems swung into effect – including an electro-optical telescope.
It saw an aircraft. It wasn't the aircraft producing the mirror echo. It was, in fact, much further away along the same line of sight. But the computerised telescopes didn't know that.
More automated systems kicked in. Other telescopes swung into action, as did high-resolution radars. Human operators were called into action – just as the "mirror track" faded.
Some thought they saw a "slow-moving blob" among the scattered clouds. Others saw nothing.
All were furiously scanning the sky for something that wasn't there.
It had taken just a few seconds. But every step in the chain of command was playing it safe.
"No matter how good the technology, no matter how well sensor systems are integrated, there's always going to be a time factor," Col. Leighton told CNN. "If the unidentified radar image is on a direct path to the White House or the US Capitol and it's only a few miles … commanders are going to make a split-second decision – especially if they don't have a positive ID of the target.
"The tendency after 9/11 is to react first and assume the worst."