Tokyo (AFP) - The twists and turns in the case of auto tycoon Carlos Ghosn have gripped Japan and the business world since his stunning arrest last month.

And just as it seemed he could be freed on bail in time for Christmas came another unexpected development: prosecutors hit him with fresh allegations that could keep him detained well into 2019.

The complex and unusual Japanese justice system has come in for criticism, especially internationally, as the case of one of the world's best-known executives has put it under the microscope.

What exactly is Ghosn accused of?

Prosecutors are investigating three separate lines of enquiry:

1) He is accused of under-reporting his salary by around five billion yen ($44 million) between 2010 and 2015 in official documents sent to shareholders.

2) He is accused of also under-reporting his salary by a further four billion yen between 2015 and 2018.

3) He is accused of passing off 1.85 billion yen in personal investment losses during the financial crisis to Nissan. He is also accused of wiring $14.7 million from a Nissan subsidiary between 2009 and 2012 for his benefit.

Authorities have pressed formal charges over the first accusation but not yet over the other two.

Ghosn reportedly denies the allegations.

Why is he still in prison?

Under Japanese law, a suspect can be held for several weeks as prosecutors probe different allegations.

After the initial arrest, they have 48 hours to question the suspect. After this, they have to request a 10-day extension from a court. They can then apply for one more 10-day extension.

At this point, they have to either press formal charges, release on bail or come up with new allegations.

In Ghosn's case, he was arrested on November 19 over the first set of allegations (hiding five billion yen over five years). Prosecutors were granted two 10-day extensions to investigate this accusation, bringing them to December 10.

On this day, they formally charged him over these accusations -- kicking off a period of pre-trial detention -- and simultaneously "re-arrested" him over the second set, effectively restarting the clock.

They obtained extensions to probe the second set of accusations until, in an almost unheard-of move on Thursday, the court suddenly rejected their request for a further 10 days.

"The court thought, perfectly rationally, that the investigators should have had enough time to gather enough evidence over these suspicions which were essentially the same as the first set for which he was charged," said Yasuyuki Takai, a former investigator.

This raised his hopes he could be released on bail, possibly as early as Friday.

But in another twist, prosecutors slapped him with the third set of allegations and the whole process starts again.

What happens next?

The clock resets and prosecutors have an initial period of 48 hours to question him -- now over the third set of allegations.

They can then request a 10-day extension, which they appear more likely to obtain as the nature of the third set of allegations is very different to the first two. They can then apply for a second 10-day extension, taking them well into January.

In addition to being detained while prosecutors probe the case, he is also in pre-trial detention following the formal charges over the first set of allegations.

He could be formally charged over the second set of allegations, meaning he would be under a second "layer" of pre-trial detention. This runs for two months, renewable every month.

When the 22-day period runs out for the third set of allegations, he could be formally charged with those or re-arrested on a fourth series of alleged crimes.

In practice, however, there are rarely more than three sets of arrests but this case has thrown out most precedents.

Could he be freed?

Not for the next 48 hours. If the court springs another surprise and rejects prosecutors' request for an extension, he could apply for bail -- which itself may or may not be granted.

The bail amount is fixed by the court and could be more than one billion yen based on previous similar cases. The current record is two billion yen.

For the first two accusations, he risks a fine of 10 million yen and 10 years in prison although he could have this sentence suspended, according to experts.

The third set of accusations is "serious," according to former investigator Takai.

If he is convicted over this, the sentence is the same but is more likely to result in time behind bars.