Unlike most UFO stories, this one appears to have substance.

On October 25, a strange craft was seen — in broad daylight — flying amid the heavy traffic of the United States’ air corridors above the state of Oregon.

Pilots radioed in reports of an aircraft flying outside registered flight plans. It was not responding to radio calls. It had no collision-avoidance transponders. But it was always just outside clear sight.

On the ground, air traffic control was also seeing strange things. Its radar was intermittently tracking an unregistered object moving at unusually high speeds.

It was cause for real concern.

After all, 9/11 showed the potential havoc aircraft flying “dark” could achieve.

So F-15 interceptor fighters from the U.S. Air Force were scrambled to take a look.

The story was first picked up by "The War Zone" blog of the automotive website The Drive. It tracked down comments from the pilots that had seen something strange that day. It also obtained confirmation — of sorts — from the U.S. air base that launched the fighters.

Now "War Zone" has obtained ─ through a freedom of information claim ─ a small mountain of documents and hours of audio recordings detailing Oregon’s air traffic controllers' actions.

Amid the accounts of phone calls, radio exchanges and pilot interviews is an enticing picture of what a substantive UFO report looks like, and how authorities struggle to make sense of what is going on above them.

‘THAT LOOKS CRAZY’

The unidentified flying object was first detected tearing through the air above Northern California by radar stations in Oakland. It was 4:30 p.m. It was unexpected. It was traveling “very fast at 37,000”.

It wasn’t supposed to be there.

At this point the recordings reveal the U.S. military was also aware of the strange aircraft. Air traffic controllers are told the Air Force was examining the radar track.

Then the unknown flying object did something potentially dangerous.

It took a sudden turn into a crowded stream of commercial airliners.

There it disappeared from radar.

But not from sight.

Startled commercial pilots began calling in reports.

Concerned and confused, for the next 30 minutes pilots and controllers tried to make sense of what was going on.

The audio recordings tell the tale of an obviously bemused controller responding to pilots. He directs other pilots on where to look. He asks if any of their air safety proximity sensors were registering it.

The military was also in the loop: references to elements of the US air defense command NORAD can be heard — “WADS” and “Bigfoot”.

Fighters are ordered into the air from the McChord Air Force Base in Washington.

What was the aircraft? Where was it going? What was it doing?

All they had to go on was that it appeared to be big. It was colored white. It was flying at about 37,000 feet.

It was now moving about the same speed as commercial airliners. It was not on radar, and was emitting no signals.

It never strayed closer than the edge of visual range.

“STEALTH MODE OR SOMETHING”

The radar and audio recordings reveal the F-15 interceptor fighters took to the air out of Portland. Dubbed “Rock” flight, these fighter aircraft are just some of those kept at a high alert status around the United States for incidents such as these after 9/11.

Strangely, they head south even as reports from commercial airliners indicate the strange craft was to the north.

One pilot calls air traffic control for an update: what’s going on?

The controller responds the UFO must be in “stealth mode or something”.

But by now it has slipped out of sight.

Losing touch with such an unregistered aircraft is no small thing.

Would it suddenly appear diving into the heart of a nearby city?

The scars of September 11 run deep.

So the urgent phone calls started.

MAINTAINING MYSTERY

Here the War Zone notes some unusual aspects of the Federal Aviation Authority recordings released under freedom of information requests.

“There were a few strange areas where conversations went mute and it’s not clear if this was edited or just an anomaly,” the blog reports.

Perhaps unsurprisingly, the redacted components appear to be responses to requests for information about the military’s activities.

“When the Manager In Charge is asked if he was asking for military assistance by another FAA controller, the tape goes blank,” War Zone reports. “The same inquiry is heard moments later, and it goes silent again before another call begins.”

And again later, another “blank” occurs as pilots are asked about what they saw.

During a call with United 612 there are some odd “dead” moments in the audio, but the pilot is heard describing the encounter, stating that he was too far away to make out the type.

The pilot of Southwest 4712 was a little more forthcoming.

“This was a white airplane and it was big. And it was moving at a clip too, because we were keeping pace with it, it was probably moving faster than we were.”

FALLOUT

The recordings continue long after the strange craft slips out of sight.

Seattle’s air traffic control manager in charge of operations urgently interrogates controllers and three of the airline pilots that reported visual contact — as well as air traffic security and an safety officers.

Everyone had to submit written reports.

Had they responded correctly?

Should the commercial airliners have been ordered to keep the strange aircraft in sight?

Was the unknown flying object a threat, and should the airliners have been ordered to scatter?

Whatever the outcome, one air traffic controller hit the situation on the head with one off-the-cuff comment:

“I have a feeling someone is going to go through this with a fine-tooth comb.”

POSSIBILITIES AND PROBABILITIES

It’s tempting to immediately make the mental leap to aliens, but extraordinary claims require extraordinary evidence.

Occam’s razor makes the fair point that when seeking an explanation for something unusual, choose the one that makes the least assumptions as being the most likely.

So what could such an explanation be?

Oregon is in the country’s northwest. It’s alongside the state of Nevada. Nevada is the site of the ultra-secret U.S. Air Force testing facility, Area 51 (or more officially Groom Lake).

We know the U.S. Air Force is fast-tracking development of its next generation stealth bomber, the B-21 “Raider”. We also know that absolutely everything about this aircraft — including its cost — is top secret.

And that’s probably one of the least secret projects being worked on at Area 51.

But a test pilot first allowing to be detected on radar and then entering a highway of commercial airliners in plain sight, is unprofessional in the least.

And the U.S. is no longer the only nation with stealth technology. Russia. China. Both have caught up and — in an increasingly belligerent world — are likely to “send messages” through overflights such as this.

So when it comes to the idea of ET taking a wrong turn at Albuquerque, it beggars belief that such a super intelligence capable of traveling vast interstellar distances would let itself be seen — if it didn’t want to be.

This story originally appeared in news.com.au.