If Britain hadn’t voted to leave the EU, and Trump hadn’t won the US election, it’s unlikely anyone outside Nigeria would have given a second thought to what went on during its presidential election campaign three years ago.

But the 20/20 vision of hindsight casts a very different light on the events of early 2015, and a campaign that now seems to eerily prefigure what happened in the US a year later. Many of the same characters, some of the same tactics.

At the heart of it all – data analytics company, SCL – the parent company of Cambridge Analytica. It had been hired by a rich Nigerian who supported the incumbent, President Goodluck Jonathan.

“It was the kind of campaign that was our bread and butter,” says one ex-employee. “We’re employed by a billionaire who’s panicking at the idea of a change of government and who wants to spend big to make sure that doesn’t happen.”

This was a standard variation on what SCL had done around the world for 30 years – this time, with a twist. Weaponising information to harm an opponent was standard methodology.

It was a methodology honed and developed in the company’s defence and military work – the fifth dimension of warfare, defined by the US military as “information operations”.

What was new, or at least new to those employees who have now spoken out, was bringing these techniques to the company’s election work.

Seven individuals with close knowledge of the Nigeria campaign have described how Cambridge Analytica worked with people they believed were Israeli computer hackers.

The sources – who spoke to the Observer over many months – said the company was looking for “kompromat” on Muhammadu Buhari – at the time, leader of the opposition.

They said the hackers offered Cambridge Analytica access to private information about Buhari.

Their testimony paints an extraordinary picture of how far a western company would contemplate going in an effort to undermine the democratic process in a country that already struggles to provide free and fair elections.

Their claims are disputed by the company, which insists it did not take possession of or use any personal information for any purpose and did not use any “hacked or stolen data”.

The company confirmed, however, that it had been hired to provide “advertising and marketing services in support of the Goodluck Jonathan campaign”.

That work seems to have come about through Brittany Kaiser, a senior director at Cambridge Analytica, who would go on to play a public role at the launch of Nigel Farage’s Leave.eu campaign, and a senior strategist on the Trump campaign.

Regarded by colleagues as a prolific networker, in December 2014 she was introduced to a Nigerian oil billionaire who wanted to fund a covert campaign to support Jonathan.

The billionaire wanted total discretion.

An ex-employee said: “(Kaiser) got a phone call. It was just before Christmas and she flew out to meet them in Washington DC. It was all a bit ridiculous. It was only six to eight weeks before the election and they were looking to spend nearly $2m.”

The election was a big deal. At stake, the future of the most populous country in Africa, and potential access to its lucrative oil reserves. The sitting president was favourite to win, though Buhari was doing unexpectedly well.

Not least because his team had hired AKPD, once the firm of former Barack Obama strategist David Axelrod, which was pushing a slick, social media heavy Obama-esque message of hope.

“There were a lot of scared millionaires worried that Buhari would get in. It was all very last-minute. A team flew out to Abuja and put together a communications campaign. It was a straightforward, normal comms campaign in most respects,” the employee said.

Most but not all respects. The Observer has obtained an astonishing and disturbing video that Cambridge Analytica used in the campaign.

“Coming to Nigeria on February 15th, 2015,” the voiceover says in the manner of a trailer for a Hollywood movie.

“Dark. Scary. And very uncertain. Sharia for all.” And then it poses the question: “What would Nigeria look like if sharia were imposed by Buhari?”

Its answer to that question is certainly dark. And scary. It’s also graphically, brutally, violent. One minute and 19 seconds of archive news footage from Nigeria’s troubled past set to a horror movie soundtrack.

There are scenes of people being macheted to death. Their legs hacked off. Their skulls caved in. A former contractor said: “It was voter suppression of the most crude and basic kind. It was targeted at Buhari voters in Buhari regions to basically scare the shit out of them and stop them from voting.”

If Buhari wins, the film warns: women would wear the veil. Sharia law would be introduced. And the inference is, you may be macheted to death.

It wasn’t just videos spreading fear. The Cambridge Analytica campaign team in Nigeria were jumpy too.

“It felt risky, being there. There were various points when we were told we were in danger.” And in the Abuja hotel to which the team was confined in early 2015, rumours abounded.

The tales are Graham Greene-esque. The hotel was where slick western consultants, including a team from the now disgraced Bell Pottinger, partied with their Nigerian counterparts. Mingling among them, western intelligence operatives – state backed, or privately commissioned, nobody was quite sure.

And then there were the meetings: three sources have told the Guardian about one that took place between Cambridge Analytica employees and two people they were told were Israeli intelligence operatives.

“There was a two-hour meeting that took place in the hotel lobby between two senior campaign members and Israeli intelligence. After which they swept our hotel rooms for listening devices and said they would switch out our phones. The story we were told was that there were intelligence agents from a number of different countries, including Israel and France, who were supporting Goodluck Jonathan and helping the campaigns.”

There is no suggestion that Jonathan was aware of or implicated in this support. Another employee said: “Basically the Israelis didn’t want (Buhari) to win.”

Other employees questioned whether they were “real” Israeli intelligence operatives, or Israeli private contractors.

A few weeks later, as the campaign was drawing to a close, there was another meeting at Cambridge Analytica’s London office.

An expert had flown in from Israel with a laptop, sources say.

And Alexander Nix, Cambridge Analytica’s now suspended CEO, and Kaiser, asked employees to take a thumb drive and download the contents on to their own computers.

The content was private emails and the information, they were told, related to Buhari’s financial and medical records.

One employee who was present at the London meeting said he had initially assumed the visiting expert was Mossad or Israeli intelligence passing on what he called “legitimate information”.

But he began to realise this wasn’t the case, he said, when he saw the reaction of his colleagues. One of them had “freaked out”, he said. “He was like, ‘What the fuck? I don’t want anything to do with this.’”

The witnesses are clear – at least in their own minds. The information they were shown had come from hackers.

Back in Nigeria, the team still on the ground found out what was going on from their colleagues in London. There was more “freaking out”. This time with live, pressing concerns.

“They were fucking scared,” said a colleague who spoke to them while they were in the country. The campaign fixer, the person with local knowledge who navigated them through the ins and outs of Nigerian politics, made it clear to them: they needed to get out of the country right away.

Cambridge Analytica had put them all in danger, they said. If opposition supporters found out, there was no saying what might happen.

One member of the team missed his flight and instead of asking the office to re-book it, he got the first fight out – to Dubai – and put it on his credit card. “Everyone just wanted to get out as soon as possible.”

A spokesman for the company said its team remained in country throughout the original campaigning period had “left in accordance with the company’s campaign plan”.

“Team members were regularly briefed about security concerns prior to and during deployment and measures were taken to ensure the team’s safety throughout.”

There are multiple wider political questions about what went on in the Nigerian election of 2015 and the role western powers played. Whether western political campaigners taking lucrative foreign contracts are contributing to the democratic framework of developing countries – or helping to destroy them. If they’re experimenting with methods and techniques that they later re-import back to our more developed democracies.

Christopher Wylie, the Cambridge Analytica whistleblower who spoke to the Observer, called it “post-colonial blowback”.

“The west found a way of firehosing disinformation into weak and vulnerable democracies. And now this has been turned back on us. This really is about our chickens coming home to roost.”

Another said: “Everything the company did after the Mercers got involved was about refining a set of techniques that they would go on to use in the US elections. These campaigns in other countries were experiments. They worked out how to harvest data and weaponise it. And they got steadily better at it.”

And what comes across most strongly, the sources say, is how little thought, if any, the senior directors in London had given to their employees and colleagues who became caught up in the activities, many of whom were in their early to mid-20s.

One member of staff who met the Israelis in the office on another occasion described them as “special forces” types. He said: “They were cliche alpha males with a certain intellect. Looked military, very composed. They looked like they could beat the crap out of you.”

Three years on, there is still stress in some of their voices when they recount these stories. Stress and fear and anger – about the danger they put in, and the lack of care shown toward them, the morally compromising position they were put in, the lack of knowledge they had about what sort of the company they would be working for when they took their jobs.

It’s why, despite the personal risks, so many of them agreed to speak.

“When I took that job, I did not sign up to any of this,” said one. Three years on, he is still angry and shocked and fearful. “You don’t know what this company is capable of,” he said.

In a statement, SCL Elections, the parent company of Cambridge Analytica, confirmed it had been hired in December 2014 in support of the Jonathan campaign.

“We can confirm that SCL Elections was hired in December 2014 to provide advertising and marketing services in support of the Goodluck Jonathan campaign.”

Asked specifically about the meetings in which staff described being asked to transfer personal information that they believed had been hacked, the firm said: “During an election campaign, it is normal for SCL Elections to meet with vendors seeking to provide services as a subcontractor.

“SCL Elections did not take possession of or use any personal information from such individuals for any purposes. SCL Elections does not use ‘hacked’ or ‘stolen’ data.”

•Text (excluding headline) courtesy of The Guardian (UK).

Update news