The crash that has forced Fernando Alonso to miss the season-opening Australian Grand Prix has still not been fully explained, and that void of knowledge has been filled ever since by a number of theories of various degrees of credibility.
McLaren initially described it as a "normal testing accident", but it very quickly became apparent that it was anything but.
Although head injuries are always a possibility in any F1 crash, it is certainly not normal for a driver of Alonso's calibre to crash on a pre-season test day, spend three nights in hospital and then miss the first race.
But just because Alonso had a nasty accident and is missing a race does not necessarily mean there is anything further amiss. So what do we know - and not know - about Alonso's crash?
What do we know about the accident?
There have been conflicting reports about the speed of Alonso's accident, which happened on the exit of Turn Three at the Circuit De Barcelona-Catalunya. But some facts are known.
This writer has seen the GPS and lap-time data of both the lap on which the Spaniard crashed and the lap before. The computer telemetry from the car, which contains all the data, including braking and steering inputs, has not been released by McLaren.
The GPS and lap time data shows both the line Alonso was on, and the speed he was doing at the time. It reveals the following:
Alonso's previous lap was the fastest he had done that day. On the subsequent lap, the lap of the crash, he brakes deeper into Turn One, and is wider out of it.
That dictates a different line through the kink of Turn Two, from where he takes a wider entry - a potentially faster line - into Turn Three, the racing line of which sees the drivers run right out at the edge of the track throughout the second part of the corner.
Alonso enters Turn Three noticeably faster than on the previous lap. It is hard to be absolutely sure of the difference, but it is in the region of 5-7km/h. In other words, a significant amount.
|Alonso in F1|
|Was the youngest driver to start an F1 race for Minardi in 2001||Last won a race in Spain in May 2013 for Ferrari|
|Won two titles with Renault in 2005 and 2006||Won four races during his previous season with McLaren in 2007|
He maintains his wider line throughout - so will have had more of the car on the kerb than on the previous lap. He loses control at a speed of about 215km/h (134mph), give or take a km/h or so.
This is about the speed a driver would be expected to be travelling at this point. Some sources say it is a bit slow, but we know it was faster than the previous lap, which was his fastest so far, and we also know he was slightly off line.
McLaren say Alonso lost control because his outside rear wheel was on the Astroturf on the outside of the corner, and also that strong gusting winds - about which the driver had been complaining all morning - may well have played a part.
The car then decelerates heavily throughout the period before Alonso hits the wall.
Engineers who have seen the GPS data say the deceleration trace is almost identical to that for braking into Turn Four, a hairpin.
Alonso did not run at a specific lower speed for any significant amount of time. There is a small blip in the data where the deceleration levels out, but this is for only between 0.2-0.3 secs.
It is impossible to be sure from the GPS data at what speed the car was travelling when it hit the wall.
McLaren chairman Ron Dennis says: "There were two G readings, when his head hit one side of the cockpit and then the other side of the cockpit."
What don't we know about the crash?
Some aspects of the accident are unexplained, most importantly why Alonso lost control of the car, and what happened between that moment and him hitting the wall.
McLaren have put this down to him being off line and the windy conditions.
There has been speculation Alonso had some kind of medical condition in the car that caused him to lose control.
But he was certainly conscious, because he was braking at maximum force - equivalent to 75kg or more - and changing gear.
The braking appears to rule out an electric shock or another kind of seizure that would have restricted his movement. And the doctors are said by McLaren to have found no evidence of any such thing.
However, Dennis has said Alonso's team-mate Jenson Button has seen the telemetry from the car and found it unusual.
"Every single input into the car was normal," Dennis said. "Changing down gear, steering, braking… there is nothing we can see that is abnormal.
"Jenson looked at the figures and said: 'Well, that's a bit strange.' But what's a bit strange when you are wrestling with the car being destabilised by the wind?
"Yes, we can guess. Yes, we can surmise and speculate. But the only thing we can comment on is the actual facts as we know them."
One highly-experienced engineer said: "Nothing I can see in the GPS data makes me think it was anything other than him going too fast and losing control."
However, the engineer added it was "a bit odd" that Alonso would suddenly slam the brakes on in this manner if he had lost control.
It remains unexplained why a driver who was conscious and braking could not also avoid the car hitting the wall.
However, Lotus driver Pastor Maldonado had an accident during qualifying at last year's Spanish Grand Prix that was very similar to what McLaren say happened to Alonso.
What do we know about the aftermath?
Alonso was briefly unconscious when rescue crews reached him. The length of this period has not been verified, but Dennis says it was "seconds".
Alonso was in a confused state when he regained consciousness, to the extent that he did not know he was a Formula 1 driver or why he was in the car.
He was sedated at the circuit medical centre and flown quickly to hospital in Barcelona. He was diagnosed with concussion but all brain and body scans showed no indication of any abnormalities.
There was no evidence of electric shock, of epilepsy or any kind of seizure, or any other medical condition that might have explained why he had lost control of a car that McLaren say suffered no problems.
He did not remember the accident and it took some time for his awareness of who he was and important aspects of his life to return.
This is called retrograde amnesia and is not abnormal in patients who have had a severe concussion.
Dennis held a news conference on the Thursday after the accident at which he said Alonso was "not even concussed". This was inaccurate and a mistake.
Alonso was kept in hospital for three nights - at the upper limits of what would be expected for a concussion, doctors say - and then returned with his parents to the family home in Oviedo in northern Spain.
On Thursday, 27 February - five days after the accident - Alonso released a video on YouTube in which he said he was "completely fine" and was looking forward to being in the car again "very soon".
Five days later, on Tuesday this week, McLaren announced that Alonso would not race in Australia on medical advice to avoid the risk of 'second impact syndrome'.
This is where a second concussion before the first is healed can have serious consequences, potentially including raised pressure inside the skull, prolonged coma and even death.
Alonso said shortly afterwards on Twitter that he was sad to miss the first race but "understood the recommendations".
"A second impact in less than 21 days 'NO'," he wrote.
Gary Hartstein, F1's chief medical officer from 2005-12, says this decision reflects the maturity of both Alonso and F1 when it comes to head injuries.
"He knows about concussion," Hartstein told BBC Sport. "He takes it seriously. We've been at this subject for so long."
Hartstein added: "So far, I don't find it odd. The timescale in terms of missing Australia is at the upper limit of everything, but it is consistent with a three-day stay in hospital. Therefore it makes sense for it to be a bit longer to get back in the car."
What do we not know about Alonso's recovery?
Alonso and McLaren both say he intends to drive in the second race of the season in Malaysia. But Alonso will still need clearance from his own doctors, and he will also be subject to tests by the FIA's medical team in Malaysia.
He will have a full neurological assessment, including what is known as an Impact test to evaluate recovery from concussion. Only if he passes these will the doctors say he is fit to race.
Beyond that, there are relatively few hard and fast rules when it comes to the handling of concussion injuries in F1 - partly because recovery varies so much from patient to patient.
Hartstein says: "A second episode of concussion within a month of the first is a month out - that's the only definitive test we use. And if they have a concussion on a race weekend, they will not be driving again that weekend."
As for the unfounded theories about what may or may not have happened to Alonso to cause the accident, standard practice in F1 is to base decisions on the results of medical tests.
In other words, it is not normal practice for the doctors to concern themselves with the causes of the accident.
However, if the FIA's investigation into the accident throws up any issues of concern, they have the right to ask further questions, including to demand the driver's full medical records.
Could a medical issue have caused the crash?
Apart from one exception, Hartstein says the chances are "extremely slim" of the crash being caused by Alonso suffering some form of cerebral or physiological abnormality.
Of the possibilities that could cause a driver to act in such a way in the car, he says: "Hypoglycemia? He's never had it in his life before, so why would it happen on his second flying lap?
"Epilepsy? The fact he was braking is inconsistent with that.
"Some kind of loss of vision that meant he could not see but could still use his limbs? A bit of a stretch; really hard to imagine."
That, Hartstein says, leaves arrhythmia - an irregular heart rate - which can be tested for. And something called carotid sinus hypersensitivity.
This is where pressure on a certain point of the carotid artery - the main one in the neck supplying the brain - causes an extreme drop in heart rate and therefore blood pressure, and can cause someone to pass out.
This, Hartstein says, is "surprisingly common", and certainly could not be ruled out for someone going through a high-G corner wearing an F1 driver's head and neck support.
If Alonso is found to be suffering from this, he would need a pace-maker to be fitted.
But, again, you cannot brake at maximum force in an F1 car if you are unconscious.
What if Alonso misses more races?
While missing Australia is understandable in the circumstances as they have been made public so far, any further absences would not be, Hartstein says.
"That would be head-scratching time, for sure," he says. "It would be highly unusual - so much so that they'd have to say something (about what was wrong). But for now concussion is the most likely and it explains everything we know at the moment."