French police are searching for a small blue Fiat that may have played a role in causing the automobile crash that killed Princess Diana and two others, police sources said today.

Investigators searching the tunnel in which the Mercedes-Benz S-280 crashed at high speed early on Aug. 31 found shards of taillight covers from another car more than a dozen yards before the spot where the Mercedes violently slammed into a pillar. Also killed were Diana's friend, Dodi Fayed, and Henri Paul, the Ritz Hotel employee who was driving. Tests later determined that Paul's system contained more than three times the legal limit of alcohol, plus traces of two antidepressant drugs.

The sources emphasized that the only link was the "placement" of the taillight-casing pieces. Investigators have determined that they came from a two-door Fiat Uno, and they were found near fragments of the Mercedes' headlight. Such fragments could be consistent with the Mercedes bumping another car from behind. In addition, the right side of the black Mercedes shows traces of blue paint, though it has not yet been determined whether they were a result of the accident or were already on the car. Fiat stopped manufacturing the Uno in 1994, though the year of the car in question is not known, police said.

Witnesses have reported hearing a long and loud screech of brakes in the tunnel before the thunderous impact. Although Paul's driving was impaired by the quantity of alcohol he consumed, the possibility that a car in front of the Mercedes caused Paul to brake suddenly has never been ruled out.

The road going down into the tunnel shifts to the left just before the entrance, so a car traveling slowly inside would not be seen by a fast-moving car behind it until the last moment. In addition, a feeder road comes into the tunnel from the right; cars that enter from that direction would not have much time to build up speed.

Another possibility, raised frequently by the Ritz and spokesmen for its owner, Dodi's father, Mohamed Fayed, is that a vehicle belonging to one of the photographers following Diana and the younger Fayed that night blocked the Mercedes, either intentionally or inadvertently.

The man who may be able to tell exactly what happened that night is beginning to communicate. Trevor Rees Jones, Fayed's bodyguard, was in the front passenger seat and has been hospitalized since the accident with serious injuries, particularly to his face. This week he regained consciousness following extensive surgery, and sources said today that he will speak with the two French magistrates investigating the accident on Friday.

"Trevor . . . can now sit out of bed for a few hours a day and has taken his first steps," his parents, Jill and Ernie Rees Jones, said in a statement issued today by the British Embassy. They said their 29-year-old son "is expected to make a complete recovery in time." There have been reports in the French press that Rees Jones, a burly former paratrooper, cannot remember the events of that evening, but they have not been confirmed.

Nine photographers and a motorcycle driver have been placed under investigation for their roles in the accident. Nearly all have spoken out, either directly or through their lawyers, and said they were nowhere near the car when it crashed. One small car belonging to a photographer, a gray Peugeot 205, was found parked in front of the Mercedes, but it showed no signs of damage, and its owner, photographer Fabrice Chassery, said he arrived after the crash. CAPTION: Curious passersby walk in the Paris tunnel where Princess Diana and two others were killed in a car crash on Aug. 31.