WHEN Ayrton Senna passed the pits at Imola for the final time at 17 minutes past two o'clock on the afternoon of May 1st, 1994, a burst of digitised information was leaving the data processors mounted on his howling Williams-Renault. This information was to represent the last reliable evidence of the Brazilian's racing career.

Caught by the tiny radio dishes on the wall in front of the Williams pit and instantly relayed to the computer screens of the technicians in the rear of the garage, it told the story of the sixth lap of the San Marino Grand Prix.

Sensors had recorded the behaviour of virtually every component of the car during the preceding minute and a half. The temperatures, the speeds, the pressures, the volumes, the wear rates on vital components. Less than a quarter of a minute later, as Senna's car smashed into the concrete wall on the outside of the long, fast left-hand curve called Tamburello, the whole lot was rendered meaningless.

The accident took 1.8 seconds from start to finish. That speck of time will be the focal point of the trial that begins at Imola on February 20, the announcement of which led to threats that the Formula One circus might not feel inclined to pitch its tents in Italy if an accidental death at a sporting event could pose a legal threat.

What happened remains a complete mystery. The accident may have had its beginning and ending within those 1.8 seconds, or - a view that the Imola tribunal is less likely to consider - it may have started with incidents taking place several weeks earlier, at the start of the season.

Senna ended the sixth lap at Imola with his car back up to racing speed after a period of five laps behind the safety car following a starting-grid accident. When the race director decided that the circuit was clear of debris and the safety car drew off the track, Senna accelerated away towards Tamburello at the front of the field.

One lap later, as he headed back into Tamburello, he was still just ahead of Michael Schumacher's Benetton. There is no doubt that Senna was troubled by the threat from a new rival, for it maintained a pattern set during the opening two races of the season, when the Brazilian had been shocked by the speed of the young German in what should have been a significantly inferior car.

Joining Frank Williams's team at the start of the year, Senna was dismayed to discover that the 1994 car was nothing like the magic-carpet machine that had carried Nigel Mansell and Alain Prost virtually unchallenged to consecutive championships. The new version was unstable over bumps and ripples, unpredictable in fast corners and poor in slow ones.

At the first race, on his home track at Sao Paulo, Senna fell behind when Schumacher's pit crew saved a vital second on each stop. Trying to catch up, Senna spun off while accelerating out of a slowish corner in what looked, from a distance, like a novice's error.

It was only later in the season that significant doubts began to emerge about the Benetton team's tactics, including their use of computerised driver aids hidden inside the transmission software of Schumacher's car - devices that had been explicitly prohibited - and the illegal removal of a safety filter from their standard-issue refuelling rig in the cause of speeding up the flow of petrol.

Some of this had begun to dawn on Senna in Brazil. At the Pacific Grand Prix three weeks later he confirmed his suspicions. Rammed out of the race at the first corner by Mika Hakkinen's McLaren, he spent some time watching Schumacher drive to victory, noting the difference between the behaviour of his Benetton and JJ Lehto's ostensibly identical car. What he knew was that the German had a clear advantage, and that his own engineers had their work cut out to catch up.

By the time the team arrived at Imola several important aerodynamic modifications had been made. Senna put the car on pole position for the third time in three races, but this time he felt more confident of maintaining the advantage in the race itself. It was not to last beyond the seventh lap.

Formula One motor racing is a business of minutely-calibrated margins and tolerances. The evidence of what happened during those 1.8 seconds between the beginning and the end of Senna's accident should have been easy to retrieve and analyse. Conclusions could have been drawn.

But when the right-hand side of the car swiped the wall, the impact destroyed the black box holding the data for that lap. Only very meagre and inconclusive telemetry from the Renault engineers' own information system, an on-board memory in the engine-management system, remained unaffected.

THOSE abstractions, and the shattered physical remains of the car, are the subject of a 700-page technical report that will form the agenda for the hearing that begins in Imola on February 20th, when Frank Williams, Patrick Head, Adrian Newey, the Imola officials Federico Bendinelli and Giorgio Poggi, and the race director, Roland Bruynseraede, will appear on charges of omicidia colposo, or manslaughter.

The natural reaction outside Italy has been to assume that the laying of criminal charges against Williams, Head and Newey means that the Italians believe them to be guilty of something, a feeling given further impetus by the finding of the technical inquiry, widely leaked more than a year ago, that the car's steering column had broken at the point of a recently welded modification.

British observers, not wanting to believe in the fallibility of Britain's Formula One technicians, and frustrated by the lengthy deliberations of the investigating - magistrate Maurizio Passarini - concluded that the Italians were looking for a scapegoat. Yet anger at the legal action may have been based on a complete misunderstanding of the judicial process.

The presence of the other three names on the charges suggests a very different scenario. It would be impossible to imagine any circumstances in which either Bendinelli, the director of the Imola track, or Poggi, formerly one of his executives, could be held responsible for a faulty piece of welding by a Williams mechanic. Nor could Bruynseraede, then a full-time employee of the FIA, world motor sport's governing body.

The logical conclusion, supported by a modicum of investigation into Italian law, is that the "trial" is not a trial in the British sense of the term. In other words the prosecutors have not yet decided that they know, in any definitive way, what caused the accident. The Imola trial is instead merely a further stage of the investigation, at which various people who could plausibly have cases to answer will be formally asked the relevant questions in an attempt to get closer to the truth.

That is why, at this stage, the presence of the "defendants" is not mandatory. If they wish, they may merely arm their legal representatives with the required information.

So Williams, Head and Newey will indeed be invited to supply answers to questions about the steering column, and perhaps about other technical matters. But, equally, Bendinelli and Poggi will be asked about the ripples on the inside of Tamburello, of which Senna was warned earlier in the meeting by his team-mate, Damon Hill, and about the narrowness of the run-off strip between the track and the concrete wall bordering the Santerno river.

And in his turn Bruynseraede will be there to consider the accident not as a sudden disaster but as the possible consequence of a chain of events unfolding over the entire first six and a bit laps of the race for instance, had the track been properly cleaned before the safety car was given the order to return to its parking space?

There are three and possibly four levels of hearing and appeal after this one. Were it to go the whole distance, we might be talking about a final verdict in a matter of six, eight or 10 years. And as an Italian friend of mine said: "There has been motor racing in Italy for ever, there have been many fatal accidents, and the law says that a death involving violence must always be investigated. But no one, I think, has ever gone to prison.

British reaction has evoked a degree of anger in Italy. Pino Allievi, the Gazzetta dello Sport's distinguished Formula One correspondent, wrote a powerful defence of his country's judicial system, concluding: "If the English, who have always considered us a banana republic, will not come to race at Imola or Monza any longer, so be it. Formula One will suffer from that much more than our civilised society."

Or, as my friend put it: "When a man dies, his death should be properly investigated. The fact that he was doing something he enjoyed doing, or that it carried the risk of death, has nothing to do with it. After all, your mother may go off on holiday on a plane. You both know that planes sometimes crash. But if it did, wouldn't you want to know if the plane hadn't been screwed together properly?

"In Italy, the dead person is protected by the same law as the living. You English, who think it's all right to give a driver a small fine for running over a small child, are being arrogant again."