The first time I met Matthew Caruana Galizia, he was running up and down the rows of cars at Malta international airport, brandishing a giant Toblerone bar – a gift from duty free. He had come to meet my flight from Heathrow and couldn’t remember where he had parked. The small white Peugeot, a hire car borrowed from his mother, was identical to dozens of others lined up in front of the terminal.

It was May 2017, and Malta was days away from going to the polls in a snap general election called by the prime minister, Joseph Muscat. His administration was at the centre of a growing corruption scandal.

On her blog, Running Commentary, Matthew’s mother had published a series of devastating revelations about the prime minister’s closest political allies. But voters appeared to be ready to return him to power. The hope was that coverage by the Guardian and other newspapers outside Malta might shift public opinion.

Back then, the family had been desperate to bring events at home to international media attention. Now, tragically, they have it. On 16 October 2017, Daphne Caruana Galizia was killed when a bomb placed under the driver’s seat of her hire car was detonated just outside her village home.

Her assassination shocked Europe. Six months on, 45 reporters from 15 countries have come together to continue her investigations. The Daphne Project is the first to be led by Forbidden Stories, a newly formed organisation based in Paris, which hopes to deter attacks on journalists by taking up the stories of those who have been silenced through imprisonment, harassment and violence.

The work began last November. The Guardian’s Rome correspondent, Stephanie Kirchgaessner, had been in touch with Matthew. He was in the UK with his two younger brothers and maternal aunt, Corinne Vella. The family had been warned they were at risk of further violence, and had come to an out-of-the-way country house for security training. Back home in the village of Bidnija in northern Malta, their father was under 24-hour police guard.

Kirchgaessner suggested driving out to meet them. Sitting around a table in an oak-panelled room, we talked into the night about the material Caruana Galizia had been working on when she died, and discussed their theories about who could have ordered the attack. I was struck by the strong bond between them, and their determination to secure not only justice for their mother, but proper investigations into the activities of politicians she had exposed.

A few weeks later we met again, at the Guardian’s offices. This time, we were joined by Jules Giraudat, a young documentary film maker from Forbidden Stories, and Stephen Grey, an investigative reporter at Reuters.

We agreed to join forces, and discussed which other partners to bring in. The template was already there: the Guardian had established close working relationships with newspapers and broadcasters around Europe thanks to previous collaborations, most recently the Paradise Papers, organised by the ICIJ in Washington.

This time, the sharing would be even more radical. Long-established rivalries had to be set aside so that competing organisations within the same country could work together: the Guardian and Reuters in London, Die Zeit and Süddeutsche Zeitung in Germany. In this fight to defend press freedom, our collaboration could in itself send a powerful message.

The group, by now joined by news organisations from Italy, Germany, France, Switzerland and the US, gathered in Paris in January.

We set a joint publication date. The Organised Crime and Corruption Reporting Project, based in Sarajevo, agreed to host the platforms we would need to access documents, interviews, films and photographs.

Matthew Caruana Galizia. Photograph: Shannon Stapleton/Reuters

Information was relayed to the Caruana Galizia family, and their contacts, both inside Malta and further afield, proved invaluable.

As publication day approached, we wrote jointly with questions for our main subjects, from the Maltese prime minister down, and shared their responses. Such collaborations can be a burden on those at the receiving end of letters from multiple news organisations, so it seemed reasonable to combine our queries.

In some cases, those we were writing about chose to communicate only through their lawyers. This has become an all-too-familiar tactic, particularly in the UK. Our colleagues in France, Germany and Italy benefit from much stronger protections, and are often surprised by the use of legal threats to try and deter British journalists from writing public-interest stories.

The project was a big financial commitment for the Guardian. Kirchgaessner and I worked for five months to research not only Daphne’s killing, and the subsequent police investigation, but to expose how Malta is selling its services as a backdoor into Europe for money from high-risk jurisdictions including Russia, Azerbaijan and China.

Our head of investigations, Nick Hopkins, spent hours editing and redrafting the articles. The video producers Irene Baqué and Laurence Topham spent days creating two brilliant films, retelling Caruana Galizia’s story and the events that led to her death.

On the evening of Tuesday 17 April, six months and one day after the bombing, The Daphne Project launched. In the days that followed, the investigations reporter David Pegg pitched in, as well as the Brussels correspondents Jennifer Rankin and Daniel Boffey, and Dan Sabbagh, a Westminster-based editor, monitored the political reaction.

In perhaps the most significant development so far, a special rapporteur has now been appointed by the Council of Europe to scrutinise the murder investigation. Malta is the first EU member state to be subjected to this measure.

The impact I am most proud of is more personal. Last week, Matthew came with his father, Peter Caruana Galizia, to speak at our morning news conference. In his hand was a copy of the Guardian, with Daphne’s photograph on the front page. For the first time since her death, Matthew said, he felt as though his mother had been returned to him.

• This article was amended on 2 May 2018. The Daphne Project launched on the evening of 17 April, not October as an earlier version said.

