IMOLA, Italy, Dec 15 (Reuters) - The Ayrton Senna manslaugh- ter trial comes to a close on Tuesday exactly a year after Formu- la One team chief Frank Williams and five other defendants were formally indicted. Judge Antonio Costanzo will deliver his verdict in a make- shift Imola courtroom, down the road from the circuit where a statue now commemorates the Brazilian former triple world champi- on who crashed and died there on May 1, 1994. But the likelihood is that Costanzo's long-awaited ruling will raise as many doubts as answers about the death of one of Formula One motor racing's greatest drivers. The main question that Senna's fans want answered is what really caused the driver's Williams to hurtle off the track and into a concrete wall at around 220 kph as it entered the Tambu- rello bend during the eighth lap of the San Marino Grand Prix. It is a question that many, not least Frank Williams him- self, believe may never be answered. "We'll probably never know what happened," the team chief said after giving evidence in court in October. The prosecution alleges that a poor weld on Senna's steering column snapped as the Brazilian entered the Tamburello, causing him to lose control of his car. It has also claimed that the way the track was maintained could also have contributed to the accident. The former assertion has been vigorously challenged by Williams as well as other drivers. "There was no proof that it (steering column failure) caused the accident, it must have been something else," Senna's former Williams team mate Damon Hill said. The track claim was dismissed by Imola officials and Italian driver Michele Alboreto, who said the asphalt had some problems but not enough to throw a car off line. Lawyers for Williams have indicated that Senna, a driver who rarely made mistakes, could have lost control while trying to avoid a piece of debris on the track. Three track officials were charged with Williams and Wil- liams' technical director Patrick Head and former designer Adrian Newey when the trial started last December. However, last month prosecuting magistrate Maurizio Passari- ni made a dramatic about-turn when he asked for manslaughter charges against Williams and the three track officials to be shelved "for not having committed the offence." That was interpreted as the end of the case for Williams, since it is highly unusual for a judge to find a person guilty if the prosecutor says there is no case to answer. However, Head and Newey, who did not give evidence in per- son, still face possible sentences since Passarini asked for them to be given one-year suspended sentences. He said their error had been "microscopic." Lawyers for Head and Newey have protested their clients' innocence and said the prosecutor's request was based on ``con- jecture, arbitrary data and numerous hypotheses." Under Italian law, both the prosecution and defence have an automatic right to appeal against any verdict, meaning that a definitive sentence may take years to achieve. A guilty verdict on any of those involved could also poten- tially pose problems for the sport in Italy. The world body FIA warned when the trial opened that drivers and teams might be unwilling to race in Italy in the light of the court action. Benetton boss Flavio Briatore was even quoted as saying that he would not race his cars in Italy if there were convictions for Senna's death. But Williams, asked by reporters in October whether he would boycott Italy if found guilty, replied: "I doubt it very much ... I don't fear the future." If the judge hands down a not-guilty verdict, ruling that the steering column and track were not at fault, then the ques- tion will remain. What did cause Senna's death?