A suspicious hole in a Soyuz spacecraft vented air from the International Space Station.

Now, it's the centre of a new rift between the United States and Russia.

It was only 2mm wide — but the hole had been carefully concealed. It had been plugged with a resin that slowly disintegrated in the cold dryness of space and was hidden in a corner, beneath a lining of insulation.

It set off alarms on Earth and in orbit as the International Space Station's (ISS's) atmosphere slowly vented into space.

A short time after it was discovered on August 29 last year, the recriminations began.

Moscow accused the US of deliberate sabotage.

NASA was dumbfounded.

Now, Russia's space agency — Roscosmos — says it has once and for all determined the cause of the mysterious, deliberate hole aboard the Soyuz MS-09 crew vehicle.

But it has been declared a state secret.

AIR ALARM

The first alarm sounded as the Expedition 56 crew aboard the ISS slept. Mission controllers on the ground detected a small drop in cabin pressure.

The six astronauts were under no immediate threat.

So, they let them sleep.

Only once they woke were they given the task of scouring the walls of the 21-year-old space station in search of a pinprick-sized hole.

Was it a micrometeor? Was it a stress fracture? Was it component failure?

"The leak has been isolated to a hole about two millimetres in diameter in the orbital compartment, or upper section, of the Soyuz MS-09 spacecraft," NASA officials said at the time.

"The crew are healthy and safe with weeks of air left in the International Space Station reserves," the European Space Agency added.

The Soyuz had arrived three months earlier, carrying three crew and fresh supplies. After docking, it was used for accommodation and sat ready as a potential "lifeboat" in the event of an emergency.

But the hole — found near a toilet — was a nasty surprise.

TOXIC ATMOSPHERE

It wasn't a defect. Or an accident. The hole had been deliberately drilled, filled and concealed.

In September last year, Russian space agency head Dmitry Rogozin confirmed it was a drill hole made by a person "with a faltering hand" — a reference to the drill-bit scuff marks around it.

He vowed to find out who did it, "either on Earth or in space".

With the immediate threat over, the scandal was only beginning.

Accusations flew thick and fast — a US astronaut had drilled the hole in an attempt to go home early; NASA was trying to sabotage the space station; Washington was trying to make Moscow look bad.

"I can unequivocally say that the crew had nothing to do with this," commander of the ISS' Expedition 56, NASA astronaut Drew Feustel, said at the time.

"I think it's absolutely a shame and somewhat embarrassing that anybody is wasting any time talking about something that the crew was not involved in."

OXYGEN CUT

"They have not told me anything," NASA administrator Jim Bridenstine said in response to news of the investigation's completion.

"I don't want to let one item set (us) back, but it is clearly not acceptable that there are holes in the International Space Station."

His counterpart, Roscosmos' Mr Rogozin, had told a youth science conference that his investigators had determined the source of the hole.

But this, he said, would not be released.

"What happened is clear to us, but we won't tell you anything," the State-run RIA Novosti news agency reported him as saying. "(It) was in the living quarters (of the Soyuz), it has long since burned up upon re-entry. We took all the samples. We know exactly what happened, but we won't tell you anything."

NASA, however, is blindsided.

It doesn't want to put the excellent relationship it has with Roscosmos put at risk.

After all, the United States still does not have the capability of putting an astronaut in orbit. Its Space Shuttle fleet was grounded in 2011 after the debris issues that brought about Columbia's fiery end could not be resolved.

In September last year, NASA and Roscosmos declared a temporary truce. A joint statement said they had "agreed on deferring any preliminary conclusions and providing any explanations until the final investigation has been completed."

However, it now appears Roscosmos has reneged on the deal.

HIGH AND DRY

Photos of the hole clearly show the marks of a slipping drill-bit. The hole is also perfectly circular. And there is no "flaring" about its edges as would be caused by a micrometeor punching its way through the metal skin.

It was also in an area difficult to access and assess. It was in the disposable spherical orbital module of the Soyuz which acts as a cargo bay on the way up and extra accommodation while docked with the ISS.

The crew capsule, designed to return through the atmosphere, is positioned below. The cargo module is discarded during re-entry.

Russian news agency TASS reported shortly after the incident that the damage might have been done while the craft was with its manufacturer undergoing final assembly or testing. This was under the auspices of aerospace company Energia in Korolyov, near Moscow.

"One of the possibilities is the spacecraft might have been damaged in the final assembly hangar. Or it could happen at the control and testing station, which carried out the final workmanship tests before the spacecraft was sent to Baikonur," an unnamed source told TASS at the time.

"Only those with proper security clearance are allowed to enter. Also, at the entrance to the hangar and the control and measurement station, security guards are checking all those who come and go."

The spacecraft passed pressure tests before being launched from the Baikonur Cosmodrome in Kazakhstan. But that is likely to be because a temporary plug in the hole was still fresh. After three months exposed to the vacuum of space, the material became brittle and flaked away.

But Rogozin was quick to deny reports the hole was a manufacturing accident, clumsily concealed by a fearful worker.