After the initial investigations into the deaths of Dr Bogle and Mrs Chandler had failed to identify the poison that had killed them, the four possible explanations for their deaths were examined. On the grounds that neither of them had any reason to commit such an act, suicide by either party was ruled out. This left what remain as the most popular theories, death by accidental poisoning, or murder by a third person.
The proponents of the argument that Bogle and Chandler died from accidental poisoning come up with three versions. One is that Bogle and Chandler deliberately took some drug in order to spice up their sexual activities (it should be remembered that neither engaged in intercourse on the morning that they died). The second is that someone, in order to play a practical joke, administered the poison without realising or intending that it would kill them. The third is that something in their environment accidentally killed them.
Bill Jenkings was a crime reporter for the Daily Mirror, and was said to have the best police contacts in Sydney. Detective-Sergeant Jack Bateman was one of those contacts, and was regarded as one of the best detectives of his generation. Both are now dead, but both were convinced that dog-worming tablets killed Bogle and Chandler.
Currently the favourite for being the poison that killed Bogle and Chandler, hydrogen sulfide was first considered in 1971, when it was dismissed by investigating police. A documentary broadcast on ABC television on 7 September 2006 firmly reestablished it as being a possible cause of death.
One of the favourites for being the poison that killed Bogle and Chandler, LSD was first suspected soon after they died, and by the 1980s was the favoured candidate. Tests in 1996 showed traces of LSD in the bodies of Bogle and Chandler, but a further test then failed to find any trace of the drug.
The Director of Forensic Medicine for the Hong Kong Police was convinced that yohimbine was the poison. It is recognised as an aphrodisiac.
From nerve gas to shellfish toxin, there were some unusual suggestions as to what might have been the fatal poison.
Many people believe that Bogle and Chandler were murdered. It would, on the face of it, seem much more likely that a poison would go undetected if the poison was intended to go undetected. Indeed, this line of argument makes the nature of the poison irrelevant, as it is only the identity of the poisoner that matters. Generally, in support of the murder theory, it is supposed that Bogle was the intended victim and that Chandler was simply in the wrong place at the wrong time.
Mr Chandler was suspected of the accidental killing of his wife and Dr Bogle. Many people also suspected him of double murder. The motive? Jealousy.
Margaret Fowler was the famous "mystery woman", who was called to the Bogle-Chandler inquest but never gave evidence. She had had an affair with Bogle, and was obsessed with him. Some of the police who investigated the case were convinced that she was involved.
There has been some speculation that Bogle was assassinated by secret agents, intent on stopping his research and his impending move to the USA. Alternatively, it has been suggested that he was permanently silenced after enquiring too deeply into circumstances surrounding the death of Dr Clifford Dalton. Supporters of the espionage theory point out that the FBI refuses to release 18 pages of their report on Dr Bogle, citing national security grounds.
More recently, in 2006, Geoffrey Chandler admitted to filmmaker Peter Butt that at the time of the case he was both a member of the Communist Party and one ASIO's people inside the CSIRO.
Evidence was presented at the Bogle-Chandler inquest about the exact positions of the cardboard cartons covering Mrs Chandler's body, and the suit covering Dr Bogle. They had not been photographed nor their positions recorded in any formal way, before being disturbed. While this was perhaps inexcusable in the case of Dr Bogle, the officers had no way of knowing that Mrs Chandler was dead. Had she been alive -- and her body was still warm -- then to have stopped to photograph the positions of the cardboard cartons would have been wasting valuable time.
What was not properly explained was why Mrs Chandler was so covered. Three pieces of mouldy cardboard, from a beer carton, covered her except for her leg. There were no other pieces of cardboard in the immediate vicinity, raising the question of how exactly the right amount to all but cover her happened to be close at hand.
The police officers who gave evidence at the inquest tended to the belief that Mrs Chandler had covered herself while ill and in a delusional state of mind. They would not agree that only a contortionist could have arranged pieces of cardboard over herself in the manner that they were found covering Mrs Chandler, but did agree it would be exceedingly difficult.
Mr Chandler, in his book So you think I did it, pointed out that the contact of mouldy cardboard on bare skin would be repulsive for a woman. He also questioned why, if his wife had covered herself, she had also covered her own face. To cover herself with cardboard -- as opposed to adjusting her dress -- could conceivably be explained by a deranged state of mind, but even this would fall short of covering her own face. On the other hand, another person would find it quite reasonable to cover her like this. Likewise, Dr Bogle. It would be physically impossible for him to cover himself in the way he was found. These factors suggested that a third person had covered them both.
But then there was the evidence of Dr Bogle's car. It was parked on the other side of Millwood Avenue, 150 metres away, the key behind the sun visor where Bogle normally left it. There were certainly no signs of a third person being present.
The location of the bodies itself, was speculated upon. Had Bogle and Chandler been taken ill on the journey to the Chandler home in Croydon, the logical thing to do would be to keep going. Suppose, however, that they decided to stop at Fuller's Bridge, to be ill. The dirt track would hardly be the place to head for. Besides, if taken ill, surely they would have left the car in a hurry. However, the interior of the car, including the careful placement of the key, suggests that everything was normal when the car stopped. So why stop? Against this, of course, Bogle, even if ill, could have left the key behind the visor out of sheer force of habit.
People have also suggested that it makes no sense that Bogle and Chandler would have stopped at Fuller's Bridge for a romantic liaison. This suggestion may be mistaken. It is true that, according to Mr Chandler, he had made it clear to Bogle that he would not be home. It is also true that the rubbish-strewn dirt track that they were found on was one of the least appealing places in the area, and that in another direction was a park and a much more pleasant, equally private, stretch of riverbank. However, the area was known locally as a lovers' lane, and Dr Bogle and Mrs Chandler were not the first couple to been in the area that morning. While Mr Chandler may have been right in saying that it was not the sort of place that his wife would have wanted to go, he cannot know if the same was true of Bogle.
The piece of carpet found on Dr Bogle's body was baffling. Seventeen days into the investigation it was identified as having come from the boot of Bogle's car. Two uses were proposed for it. It could have been used by the couple to sit on. Equally, Mrs Chandler might have used it to cover Dr Bogle. She was a trained nurse, and might have used the carpet in order to try to keep him warm.
But the first use was unlikely, as it was too small, at 91.5cm by 68.6cm, to sit on comfortably. Surely, too, they would have chosen a better place to sit. The second use is also unlikely, for Mrs Chandler was poisoned in the same manner as Dr Bogle, and for her to have gone to the car, fetched the carpet, and returned, sounds strange. On the other side of the bridge were houses, and other cars were parked off Lady Game Drive. To have fetched a piece of carpet rather than get help defies rationality.
This has remained the continuing problem of the Bogle-Chandler case. For every theory which has a number of clues pointing to it, there are other pieces of evidence to reject it. The eventual verdict at the inquest reflected this. The Coroner, Mr Loomes, returned the official finding that the deceased had died from "acute circulatory failure. But as to the circumstances under which such circulatory failure was brought about, the evidence does not permit me to say."
In simple English, Dr Gilbert Bogle and Mrs Margaret Chandler had died because their hearts had stopped beating, and they had stopped breathing. There was no verdict about death by Person or Persons Unknown. They were certainly dead, but why? The answer may never be known.