In small communities, stories are often told in asides and whispers. When the boat, the Ben Lomond arrived near the shores of the Isle of Lewis in May 1952, rumours began immediately. They were still circulating when I was growing up on the island in the 1960s and 1970s.

The boat was anchored off Tolsta Head on the island’s east side for much of that year. On board, local villagers could see the shrouded figures of men taking cages filled with monkeys and guinea pigs to a pontoon a short distance away from their shoreline. A short time later, a veil of mist could be viewed upon the nearby stretch of ocean as poisoned gas was unleashed to test the effectiveness of a range of biological weapons – from bubonic to pneumonic plague – on the unfortunate animals. Their corpses would later be examined to determine the success (or failure) of the experiment.

The operation was shrouded in secrecy and at the time, the islanders knew only that local fishing boats were banned from sailing into a narrow stretch of waters and that youngsters from the village were discouraged from swimming nearby. Their confusion must have only increased when the fishing boat, the Carella from Fleetwood in Lancashire sailed into the area to be greeted by the flapping of red warning flags and the sounding of alarms. It would later emerge that scientists on board the Loch Lomond had feared the Carella’s fishermen might carry the plague back to their home fishing port in the north of England.

It was a story I heard about in school. Raised in Ness, north Lewis, I stayed in Stornoway during the week while attending the local secondary and it would be whispered about in the school hostel and at the back of the bus as I returned home for the weekend.

We knew little of what had actually happened aboard the Ben Lomond, other than it was one of two such experiments off the Western Isles. The first of these – in 1952 – was named Operation Cauldron; the second, which occurred the following year both off Tolsta Head and the Flannan Isles, was called Operation Hesperus, all part of a series of similar events known collectively as Pandora.

The rest was a fog of half-truths and legends, tales that were as much guesswork as reality. There were some, for instance, who believed that the experiments involved a search for antidotes for chemical weapons rather than an investigation into how well (or badly) they worked.

A little light was shed on the exercise in the mid-1980s when some of the details involved in the Carella incident were discovered in a crumpled, highly classified file. Leaked to the Observer, the story appeared under the headline “British Germ Bomb Sprayed Trawler” in July 1985. Questions were asked about the matter in the House of Commons by the Western Isles MP, Donald Stewart. Yet even after this and the subsequent questioning of Calum MacDonald, Stewart’s successor, much of the mystery remained. It is likely that Francis Pym, the Minister of Defence at this time, knew even less about the affair than the MP who was directing questions towards him.

As a result of this vacuum, myths have grown. Among the tales I have heard is that the Carella was heading back to Fleetwood when the incident occurred. (It wasn’t. It was sailing in the direction of Iceland, fishing for cod in the cold waters there.) I have been told that cattle had to be slaughtered in Skye because of the effects of brucellosis and that the experiments caused a large cancer outbreak in Lewis the following year. (As far as I can determine, neither of these stories is true.) I was also informed that a large number of dead monkeys and guinea pigs was washed up on Tràigh Mhor, one of Tolsta’s beaches. Again, this hasn’t been verified.

Together with a short account of one of the men aboard the Ben Lomond being arrested for causing a breach of the peace, it is these stories that helped me in the creation of my novel, In a Veil of Mist. While researching and writing the book, I have also come to know more of the truths that lie behind the tales. I am aware, for instance, that Operation Cauldron was not the work solely of the British government but also its United States and Canadian equivalents; that other similar experiments occurred off the coastline of the Bahamas and less populated parts of the United States and Canada.

I have learned that the civil servant Clive Ponting – the whistle-blower who leaked details of the sinking of the Argentinian light cruiser, General Belgrano during the 1982 Falklands War – was also responsible for revealing what occurred in the Western Isles during Operation Cauldron. This was revealed when the Observer published his obituary following his death last year. Together with the release of a video in 2008 that recorded the experiments taking place, this provided me and others with much more information about exactly what had occurred off Tolsta Head than we’d possessed before.

Despite the mysteries surrounding Operation Cauldron, it has, like the anthrax tests conducted in Gruinard Island, Wester Ross some 10 years previously, left a legacy among the local population. In the early months of the Covid-19 crisis, senior British politician Michael Gove (among others) suggested that the Scottish islands were perfect locations to develop strategies to end the lockdown, claiming there was “scientific justification” for piloting measures such as contact tracing and lifting lockdown restrictions at a “progressively greater rate” than on the UK mainland.

The reaction towards his words was strong. Some pointed out that places like the Outer Hebrides had a larger percentage of older, vulnerable people among its population than most areas in the country, and that the islands had far too often been used for “experimentation”. Its people were not to be employed as “guinea pigs” in any such scheme. The choice of this language is interesting in the context of both Operation Cauldron and Gruinard. It seems people have not quite forgotten what happened in this area seven decades ago.

In a Veil of Mist by Donald S Murray is published on March 25 by Saraband, £9.99. He appears at the Ullapool Book Festival in May https://ullapoolbookfestival.co.uk