With 11 minutes remaining in Formula One's preseason testing, the FIA released a short statement saying it had investigated Ferrari's power unit and reached a confidential agreement with the Italian team. The timing, brevity and unexpected content of the email immediately set alarm bells ringing in the paddock, not least because those with knowledge of the agreement had already left the circuit in the direction of the airport.

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The statement said the details of the agreement would remain between the FIA and Ferrari, leaving a blank canvas for rivals, media and fans to unload speculation. The other nine teams had no knowledge of the investigation before the statement landed in their inboxes, and some expressed shock and a great deal of suspicion at its contents.

You could also sense a huge amount of frustration as, while the statement did not confirm Ferrari had broken any rules, it did not address the suspicions that rival teams have harboured about Maranello's power unit for over a year. Sure enough, the teams responded on Wednesday with a statement of their own questioning the Ferrari/FIA settlement.

What's left is a messy situation and a series of questions that might never receive satisfactory answers.

What do we know?

Rival teams were suspicious of Ferrari throughout 2019 Photo by Peter J Fox/Getty Images

Part of the FIA's role in F1 is to monitor the legality of all cars and power units and investigate anything suspicious accordingly. The first section of Friday's statement tells us that the FIA completed a thorough investigation into Ferrari's power unit over the winter and analysed the way in which the team was operating it.

Such investigations are not unheard of, but this one came after a great deal of suspicion around Ferrari's perceived power advantage in 2019. Ferrari has arguably had the best power unit since 2018, but its advantage appeared to increase significantly in 2019 and did so at a time when mature engine regulations were expected to bring about performance convergence.

To understand the concerns, it's important to have a grasp of how the current F1 power units work and the regulations that define them. The existing turbo-hybrid rules were introduced in 2014 and are written so as to encourage fuel efficiency. By setting a limit at which fuel can be consumed by the engine -- 100kg per hour -- the regulations challenge the teams to generate as much power from their limited fuel flow as possible.

Each drop of fuel has a calorific value, and, when it is burned, the aim is to transfer as much of that energy to the crankshaft as possible. That ratio of potential energy in the fuel to power at the crankshaft dictates the thermal efficiency of the engine, and in the early years of the turbo-hybrid regulations, F1 manufacturers found big gains, going from roughly 40% thermal efficiency in 2014 to over 50% by 2016.

But the law of diminishing returns means that the longer the rules stay the same, the harder it becomes to find gains, and Ferrari's improved straight-line performance did not fit with the gradual progress expected in the sixth year of stable regulations. Naturally, its rivals were suspicious of its advantage at the start of the year, but they initially held back from formally questioning what Ferrari was doing.

Ferrari itself said that its impressive acceleration and high top speeds were only partly down to improved engine performance and that the rest was coming from the low-drag aerodynamics of its car. But without knowing the figures from Ferrari's engine dyno and wind tunnel, there is no way of pinpointing exactly where balance the lies and how much extra horsepower Ferrari had found.

Nevertheless, rival teams were convinced there was more at play than a low-drag car, and while the power gains appeared remarkable within the regulations, they would look a lot less impressive if Ferrari had found a way around the fuel flow regulations.

One theory put forward by a rival team suggested a controlled leak of oil from the intercooler to the combustion chamber under high operating pressures, which when burned would provide extra calorific value to the existing mix of fuel and oxygen and result in more power. It would be very hard to detect as the engine would still be burning the normal amount of fuel and, because it was coming from the intercooler, it would also avoid detection when the oil consumption of the engine was measured at the end of the race. But although the FIA confirmed that such a system would indeed be illegal on any car, to this day there is no proof that Ferrari was using one.

Another theory put to the governing body was that Ferrari might have found a way to trick the FIA fuel flow sensor to allow the rate of fuel to the engine to go beyond 100kg per hour undetected. That would obviously provide more power but would require a system that either interferes with the frequency of the FIA's fuel flow sensor or works around it. Again, the FIA confirmed that such a system would be in breach of the regulations without suggesting or proving that any team had managed to do so.

The hope among rival teams was that the FIA's clarifications, issued via amendments to the regulations known as technical directives, would force Ferrari to adapt the way it ran its engine and would impact on its performance. But despite a slight dip in form at the U.S. Grand Prix, which Ferrari attributed to an increase in drag in the pursuit of more downforce, the results of the final few races proved inconclusive.

But the FIA wasn't finished. After the Brazilian Grand Prix, it seized parts of Ferrari's fuel system to continue its investigations. Not long after, a further technical directive was issued, announcing that a second fuel flow sensor would be introduced in 2020 to tighten the FIA's monitoring of fuel consumption. Again, there was no suggestion from the governing body that Ferrari had broken the rules, and it appeared to be a way of allaying fears heading into the new season.

Speaking at a Christmas lunch after the 2019 season, Ferrari team principal Mattia Binotto told journalists he believed his team had proved its power unit was completely legal.

"If I look at the whole season, we have been one of the most-checked teams -- both before and after the technical directives had been worked up," he said. "And when you have got a performance advantage -- and certainly we had it for the whole season, because we've had a power advantage compared to our competitors since the first race -- being checked is normal and it is somehow good because through the checks you are proving your legality after the TD has been issued.

"The number of checks on our cars have multiplied, the results have been shown to the FIA and the details have been discussed. So whatever could have been done through collaboration with the FIA has been done.

"We have never changed our way of operating the engine for the last part of the season, showing that our power unit is fully legal. Had that not been the case, if there would have been any non-legality, it would have come out at the very first check."

What does the agreement between the FIA and Ferrari mean?

Ferrari boss Mattia Binotto claims FIA checks confirmed his team's power unit was legal Photo by Charles Coates/Getty Images

Unknown to the media and rival teams, the FIA had continued its investigations after Binotto's statement and completed a full analysis of the operation of Ferrari's power unit earlier this year, which likely included the fuel system but was not limited to it. It appears to be a significant undertaking on the part of the FIA, but to the outside world the only tangible result of all that hard work was two paragraphs in a statement.

The statement doesn't say Ferrari's power unit was found to be in compliance but it also doesn't say it was found to break any regulations. To reach a definitive conclusion, the FIA could have taken the matter to the International Tribunal, but given the complicated nature of F1 power units and Ferrari's stated belief that it was acting within the regulations, it would have likely been a long and expensive legal battle with the potential to damage the image of F1. Add to that no guarantee of a satisfactory outcome for the FIA at the end, and an agreement with Ferrari might have seemed like a more sensible outcome.

However, the fact the agreement is being kept secret is unusual and is at the centre of the complaints from the rival teams. The lack of transparency creates more questions than it answers and that is clearly not a satisfactory outcome for Ferrari's rivals.

The second paragraph of the statement said Ferrari had "agreed to a number of technical commitments that will improve the monitoring of all Formula One power units for forthcoming championship seasons as well as assist the FIA in other regulatory duties in Formula One and in its research activities on carbon emissions and sustainable fuels". It is framed as a "settlement" rather than a punishment, but it still looks to the outside world as if Ferrari has had to make a concession in order for the situation to go away. When contacted by ESPN for details, a Ferrari spokesperson was unable to offer any further comment.

Of course, the agreement is unsatisfactory for anyone outside Ferrari and the FIA, and the question remains: did Ferrari break the rules in 2019? Rival teams are assessing their options to get an answer and greater transparency over the agreement, but the statement is not like a stewards' decision and there does not appear to be an obvious mechanism within the FIA's own regulations for it to be challenged. It could be, however, that in avoiding one legal battle, the FIA is on course for another.

The one thing that is clear is that questions raised by the investigation and the FIA's statement are not simply going to disappear before the start of the season.