The Airbus A321 was flying from Frankfurt, Germany, to Seville, Spain, in February of last year.

Full details of the incident came to light on Thursday, when Spanish investigators published their final report.

More than 200 people were on board the flight, which appeared to be running smoothly before it crossed the Spanish border.

With about half an hour remaining of the journey, the captain left the flight deck to visit the bathroom. He told investigators the first officer appeared to be in good condition at this time.

But when the captain returned eight minutes later, he was unable to get back into the cockpit.

In an interview with investigators, the first officer said he had suddenly lost consciousness.

The captain tried to enter the security door's access code five times. At the same time, a flight attendant tried to contact the first officer on the intercom.

After receiving no response, the captain used the emergency access code. Before the timer for this expired, the first officer was able to recover enough to open the door.

The 38-year-old was described as "pale, sweating, and moving strangely," so the captain called for help.

Cabin crew and a doctor, who was travelling as a passenger, administered first aid, while the captain diverted to Madrid ? the nearest airport.

Upon landing, the first officer was taken to the hospital.

His "sudden and severe incapacitation" was the result of a seizure disorder caused by a neurological condition, investigators wrote in Thursday's report.

The Spanish aviation regulator said his condition was difficult to detect ? and would appear in a medical exam only if his symptoms had been present at the time or had occurred before.

Investigators recommended that the European Union Aviation Safety Agency tell all airlines about this incident, so they could reassess the risks of one pilot being left alone on the flight deck.

Lufthansa declined to comment when contacted by Business Insider. (Business Insider Africa but headline rejigged)

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