POLICE CAST NET FOR 2ND CAR IN DIANA CRASH.

Byline: Craig R. Whitney The New York Times

In a nationwide search for a car that may have been lightly sideswiped by the Mercedes-Benz carrying Diana, Princess of Wales, just before the tunnel crash that killed her Aug. 31, French police began questioning the owners of 40,000 French-registered Fiat Unos this week.

The police acted after investigators determined that a scratch on the left side of the Mercedes bore a white paint scratch and a black fender mark that could have come from a Fiat Uno. Fragments of a taillight and rear directional signal from a Fiat Uno were also found near the scene of the crash, a tunnel under the Place de l'Alma near the Seine, but no Fiat owner has ever turned up as a witness, despite repeated police requests.

Diana and her companion, Dodi Fayed, were killed with their French driver when the Mercedes, traveling at an estimated 90 miles an hour, crashed into a support pillar of the tunnel while trying to outrun a pack of celebrity photographers on motorcycles.

Nine of the photographers and one of their motorcycle drivers were later arrested on suspicion of contributing to the cause of the accident. All said the Mercedes had left them far behind by the time it went out of control. The princess's driver was later found to be legally drunk, but officials of the French Justice Ministry said Wednesday that investigators were not yet sure they had all the elements necessary to reconstruct how the accident happened.

More than that they will not say, but officials have let it be known that if the Fiat was carrying other photographers and the driver feared having caused the crash, that would explain why nobody has come forward.

So, according to lawyers and others close to the case, the police have been looking for the Fiat themselves, concentrating first on cars registered in western suburbs of Paris where it might have been headed home at the time of the accident, after midnight early that Sunday.

``What were you doing last August 31?'' two police agents asked Julien Fernandez, a driver for Paris-Match magazine, in suburban Asnieres-sur-Seine on Tuesday. He was on vacation in Spain, he answered.

Edwige Pelmard, 30, a master of ceremonies, said she was asleep at the time.

Both their Fiats were other colors than white, but the police said the white paint scratch on the princess's car could have come from an undercoat.

Under French law, the chief investigators, Herve Stephan and Marie-Christine Devidal, cannot reveal anything about the probe until it is finished. Stephan's spokesman refused Wednesday to say even whether the investigators have carried out a reconstruction of the crash with witnesses and the 10 arrested members of the paparazzi posse present. Other officials said they have not, and will not until they are finally satisfied that they have assembled all the elements.

The one survivor in the Mercedes, Trevor Rees-Jones, a bodyguard working for the Fayed family, has told investigators he does not remember anything about what happened before the Mercedes slammed into the column.

Some motorists in the tunnel that night have told police that a car and a motorcycle appeared to try to slow the Mercedes just before it went out of control.