When Henri Paul is buried in his home town in Brittany this morning, many of the secrets surrounding the August 31st car crash in which he died with Princess Diana and her companion Dodi Fayed will be buried with him.

Could Paul, the 41-year-old deputy head of security at the Ritz hotel, have succumbed to a suicidal impulse when he drove the Mercedes S280 at high speed into the 13th column of the tunnel under Paris's Alma Bridge, as British tabloids have suggested?

Where and what did he drink in the final hours before the accident? Was he an alcoholic, or simply an average Frenchman who sometimes overindulged? What caused the depression for which he was secretly treated?

Who was Henri Paul, the man who killed Princess Diana? French police are looking for a blue Fiat Uno which Paul may have clipped when he swerved to avoid a collision, but even if the mystery car is found, that will not exonerate Paul.

In the events of that fateful night, he has been proven guilty on four counts: of drunk driving; of speeding; of mixing prescription anti-depressants with alcohol; and of driving without the required licence. In the eyes of investigators, Henri Paul bears near-total responsibility for the crash.

Such posthumous infamy is an ironic end for a man who led a life of quiet desperation. A bachelor, he had lived alone in a meticulously neat 60 sq m fourth-floor apartment at 33 rue des Petits-Champs. From 1990 until 1992 he had shared the flat with Laurence Pujol, a secretary at the Ritz, and her daughter from a previous relationship, Samantha.

"He was too protective, too paternalistic with me," Pujol told Le Figaro. "I couldn't bear the situation." After she moved out, Pujol continued to see Paul until 1995. In their last telephone conversation a year ago, he told her: "The door is still open."

Friends believe Paul started taking anti-depressants at the time of the break-up. The only photos in his apartment were of the couple and Samantha on their holidays together, and he had left her name on his mailbox.

Some of Paul's associates remember him drinking heavily a decade ago, before he dated Laurence Pujol. His closest male friends say Paul discussed neither his love life nor the rich clients he served at the Ritz. They did not know he was taking the antidepressants Prozac and Tiapride, often used to counter the effects of alcoholism.

After his death, police found the pills stashed in the Ritz's underground security room. This year Paul was passed over for promotion to be the hotel's head of security. The Ritz will not say why, but that doubtless deepened his depression.

Henri Paul was careful to hide his problems, and his drinking. Three weeks after his death, French police have still not established with certainty where or what he drank from 7 until 10 p.m., before returning to the Ritz at the insistence of the hotel's acting director.

While Princess Diana and Mr Fayed ate dinner between 10 p.m. and midnight, Paul ordered two drinks at the Ritz's Vendome bar. To have reached an alcohol ratio of between 1.75 and 1.87 grams per litre of blood, confirmed by three blood tests, he must have consumed the equivalent of at least seven glasses of whiskey earlier in the evening.

Witnesses give contradictory testimony as to whether Henri Paul was visibly drunk when he left with the princess, Dodi and his bodyguard, Trevor Rees-Jones. Alexander Wingfield, another Fayed bodyguard, claims Paul appeared sober. A jerky security camera video is inconclusive. But Ritz employees have said he was "drunk as a pig". When Paul went out to the front of the hotel several times to joke with waiting photographers, one asked out loud: "Is he drunk or what?"

Henri Paul had organised his life in such separate compartments that male friends like Claude Garrec, a printer with whom he played tennis on the last morning of his life, did not even know the name of the blue-eyed, blond art history student whom he often invited to local restaurants. Every Wednesday night he went bowling with his buddies, then dined at the Grand Colbert, whose owner says he drank "like any normal Frenchman".

According to Le Figaro, the blonde, identified only as Sylvie L, had a date with Paul when she waited for him in vain: he had died 15 hours earlier. Her concierge told her the news when she returned to the studio apartment which she and her companion, a violinist, rented from Paul. Investigators have interviewed Sylvie L at length in hopes of understanding Paul's personality. Like most of his friends, she is from Brittany and appears to have been a friend and confidante rather than a mistress.

A licensed pilot, Paul spent rare leisure hours at home on a computer flight simulation game, or playing the piano. The bartender at Le Champmesle, a lesbian bar near his apartment, says he discussed the literary novels and thrillers that he read with her. But his job at the Ritz consumed him. He took only one week's holiday this summer, in Spain with three secondary school friends from Brittany. Even then, he took his portable cellphone and stayed in touch with the Ritz.

Henri Paul was a creature of habit, whose adult life passed within a few square kilometres of central Paris's first and second arrondissements, all near the Ritz. He had lived at No 5, rue des Petits-Champs, when he moved to Paris after his military service in 1979. Already he seemed to accumulate disappointments: he dreamed of being an airforce pilot, but his eyesight was too poor.

He offered private flying lessons, but when that came to naught he spent the next six years selling catamarans at a boutique called Emeraude, also in the rue des Petits Champs. When he later moved to a bigger apartment, he stayed in the same street. Inhabited by office workers in the day, empty except for a few Asian restaurants and gay and lesbian bars at night, Henri Paul's neighbourhood is an old, if soulless, part of Paris.

Back in 1986 one of Paul's friends in the police tipped him off that the Ritz was recruiting for its security service. To get the job, he embellished his c.v., claiming to have reached the rank of captain in the air force when he was only a reserve lieutenant and falsely stating that he had been in charge of security for the Rochefort airbase.

Paul rose to deputy head of security at the Ritz, with an annual salary of £24,000 and 20 employees under his orders. He supervised the installation of security cameras in the hotel's halls and lifts and, according to one employee, even hid one in the kitchen clock.

Other employees considered Paul a spy for Frank Klein, the Ritz's director and, according to Paris Match, Paul had several employees fired for indiscretion. The magazine says that Paul is listed by French domestic intelligence, the Renseignements Generaux, as an occasional informer. Other reports say he kept an eye on suspicious foreigners for the authorities. His relations with the local police commissariat were so cosy that he got friends' traffic tickets annulled in exchange for a little foie gras from the Ritz.

If the investigating magistrate, Judge Herve Stephan, concludes that Henri Paul caused the deaths of Princess Diana and Dodi Fayed, ultimate responsibility under Article 1384 of the French civil code will lie with the Ritz's owner and Paul's employer, Dodi's father Mohamed al-Fayed.

Although the British royal family, the Spencers and the surviving bodyguard Trevor Rees-Jones, another al-Fayed employee, are not likely to sue the Egyptian billionaire for damages, the moral stigma attached to such a finding would be devastating.