PHOTOGRAPHS to back up the contention that Ayrton Senna's fatal accident in the 1994 San Marino Grand Prix was caused by debris on the Imola circuit may form a major part of Formula One team chief Frank Williams's defence in his trial for the Brazilian driver's manslaughter.

Williams, together with his technical director Patrick Head, former chief designer Adrian Newey, and three others, face the manslaughter charges. The prosecution - the case starts on Thursday in Bologna, Italy - is expected to allege that Senna's Williams lost control on the seventh lap due to a pre-impact failure of the steering column which had been modified before the race.

However, a photograph published in yesterday's Sunday Times shows Senna's Williams, just before the crash, apparently about to run over a piece of debris left after an earlier collision between J.J. Lehto's Benetton and the Lotus driven by Pedro Lamy.

A second after the photograph was taken, the Brazilian's car plunged off the circuit and he suffered fatal head injuries when his car collided with the concrete wall lining the track.

There is also evidence that Senna may have pressed too hard on cold tyres after several laps of running at a much-reduced speed behind the safety car, while debris from the earlier collision was removed from the straight in front of the pits.

Formula One tyres depend on heat built up by vigorous use to retain their optimum operating pressures and generate maximum grip. If allowed to cool, they can lose grip dramatically and in this situation, they created a situation beyond even Senna's control.

Neither Williams nor Head will attend the first day of the trial - they are not obliged to - as they wish to avoid turning the case into a media circus.