There have been many conspiracy theories surrounding the car crash. However, British and French police investigations put the blame largely on Paul for being affected by alcohol and prescription drugs, and later driving recklessly.
Paul had worked for the Fayed family for almost 11 years. He received his private pilot's licence in 1976 and was said to have enjoyed renting an aircraft to fly over to Lorient. Three days before the crash, Paul successfully passed his annual pilot's physical examination, which includes tests for any alcohol problems (including a blood and liver test). Paul's parents, Jean and Giselle, claim this test would have shown if he had serious problems with alcohol. The original Certificat d’aptitude physique et mentale was shown on Diana - Geheimnisse der Todesnacht, on the German TV channel ZDF, in 1998. However, the Operation Paget Inquiry reviewed Europe-wide standards for pilots' medicals that were in force in 1997 from the Civil Aviation Authority that suggest no specific medical test for alcoholism was undertaken, and that a self-certification of alcohol problems was required from individual pilots. Paul made no certification of alcohol problems and none were externally apparent to the doctor who examined him. As part of security training, Paul was known to have twice been to Stuttgart, Germany, on specialist courses run by Mercedes-Benz on how to handle their cars; these included anti-terrorist and anti-kidnapping evasion techniques.
On the night of 31 August 1997, Paul was under the influence of alcohol and tried to elude paparazzi photographers at high speed—estimated at over double the 50 kilometres per hour (31 mph) speed limit—when the Mercedes S280 he was driving crashed into a column supporting the Pont de l'Alma tunnel in Paris. Paul's blood alcohol content level was subsequently found to be between 1.73 g/L and 1.75 g/L (~>0.17% mass/vol.), a figure more than three times the threshold for drunk driving as defined under French law. Paul's parents dispute the authenticity and the accuracy of the test results, as does Dodi's father, Mohamed Al Fayed.
Friends of Paul's testified in statements to the French police that he did not have a remarkably high tolerance for alcohol and was seen on social occasions to drink for several hours while showing obvious signs of drunkenness. In her statement to French police, his medical doctor Dominique Mélo, who was also a friend, explained: "Henry (sic) drank like everyone else, but not to excess". "He did not have the clinical stigmata or the behaviour of a chronic alcoholic", she explained further.
Paul's doctor testified that in the two years leading up to his death he had depressive episodes about the break-up of a long term relationship and had sometimes taken to drinking at home outside a social context. She believed he was not alcohol-dependent but she was worried that he might become so, and in about June 1996, she prescribed him the anti-depressant Prozac (fluoxetine), and an anti-alcoholism medication, Aotal (Acamprosate). Traces of the anti-depressants were found in post-mortem examinations of his blood. The inquest revealed that the autopsy also found Paul's liver to be normal with no indicating signs of problems connected with alcoholism.
Operation Paget investigated the reliability of the post-mortem examinations using DNA comparison of the disputed blood sample by comparing a DNA profile from it with Paul's mother's DNA profile. The test produced a result that there was maternal relationship between the two profiles to a probability of 99.9997%. The level of carbon monoxide in this blood sample was attributed to the area of the body it was taken from and to his living in a built up urban area and smoking of small cigars in the hours leading up to his death.
It was disclosed that in November 2006 John Stevens had a meeting with Paul's parents and told them that their son was not drunk, and was found to have indisputably had two alcoholic drinks (this was verified by bodyguards Trevor Rees-Jones and Kieran Wingfield, two barmen in the bar, till records from the hotel bar and a drink bill). Five weeks later the report stated that Paul was twice over the British drink-drive limit and three times over the French limit. An expert cited in the report estimated that Paul had drunk the equivalent of ten small glasses of Ricard pastis, his favourite aperitif, before driving.
At the British inquest in February 2008, Stevens denied "deliberately misleading" Paul's parents and explained the apparent contradiction in his statements by saying that Paul did not meet the standard definition of being drunk which is dependent on observable physical behaviour. He was though clearly "under the influence" of alcohol and unfit to drive.
An unexplained prescription only drug called albendazole (or Zentel) used to treat worm infestations was also found in hair samples from Paul; this drug is said to be commonly given to homeless people living on the streets. Paul's doctor denies prescribing this drug to Paul.
In previously publicly unknown CCTV footage, which was shown to the British inquest jury on 4 October 2007, Henri Paul is seen on the night of the accident waving to photographers. Inspector Paul Carpenter who was giving evidence confirmed to the court that Paul had waved at the photographers within minutes of the couple's departure. He said that one of the photographers, sitting in his car close to where the couple would later exit the hotel, was in contact with other paparazzi. Inspector Carpenter earlier explained to the jury: "You will see Henri Paul exit into Rue Cambon [at the back of the hotel] and when you watch this sequence you will see him raise his hand as if waving to the paparazzi across the road. If you look at the paparazzi across the road you will see one of them raises his camera . . . Jacques Langevin." The images claim to cast doubt on the long-held belief that the group of paparazzi waiting outside the hotel had been acting without any help from inside the hotel.
Allegations have been made about Paul in the years following his death concerning his supposed involvement with the French Security Services, and possibly with those in the UK. The claims were investigated by the Metropolitan Police in Operation Paget headed by John Stevens over the course of three years. Chapter four of the investigative report is entirely given to the allegations about Henri Paul.
The conclusions of the 2006 Operation Paget investigation were that Paul's involvement with the Security Services was limited to low level co-operation with the French DST when high-profile guests stayed at the Ritz Hotel and he received no payment for this in line with French Government policy. It further noted such involvement with national security services is common among senior security staff at upmarket hotels in major world cities.
It was found after Paul's death that he was in possession of a large sum of money on his person and had a large personal fortune that far surpassed his expected income held in fifteen separate bank accounts. It was alleged this could only have been as a result of payment from an illicit source, supposedly a national security service.
According to his best friend, Claude Garrec, the large quantity of cash found in his pockets, FF12,565 (approximately equivalent to £1,250 or US$2,500) could be attributed to a requirement of his job to run errands for wealthy guests at the Ritz Hotel when required. A large quantity of cash would need to be on hand to perform errands at short notice as wealthy people are known to often not carry cash. Paul also received large tips for performing these errands. His mother told of an occasion when Paul received FF5,000 (£500 or US$1,000) as a tip from a relative of an Arab Prince for shopping for some luxury textiles for her. Such four-figure tips were not a rare occurrence for him during his eleven years at the Paris Ritz.
Operation Paget concluded that the cash and the money in his bank accounts were unlikely to have come from any national security service as there was no evidence in his bank accounts of any attempt to disguise any money coming from a clandestine source and there was no opportunity for him to begin to implement any plan he might have been instructed to undertake in return for payment on the night of the accident. Furthermore, he was a 41-year-old single man with no dependents who had worked all his adult life and owned property which he let out to tenants and this was a possible explanation for the FF1,700,000 (approximately £170,000 or US$340,000) that made up his personal wealth at the time of his death. The large number of bank accounts he had his money deposited in is not uncommon in France, where banks will routinely open several accounts for different purposes, all to serve one customer.