This is the final part of a guest post by Paul Lewis that originally appeared in the book Investigative Journalism: Dead or Alive? You can read the introduction here and the second part – on the investigation of Ian Tomlinson’s death – here.

Mubenga’s death had been similarly “public”, occurring on a British Airways commercial flight to Angola surrounded by passengers. As with Tomlinson, there was a misleading account of the death put out by the authorities, which we felt passengers may wish to contest. Within days, open journalism established that Mubenga had been handcuffed and heavily restrained by guards from the private security firm G4S. He had been complaining of breathing prior to his collapse. After the investigation was published, three G4S guards were arrested and, at the time of writing, remained on bail and under investigation by the Met’s homicide unit.

Our strategy for finding out more about Mubenga’s death centred on two approaches, both aided by Twitter. The BA flight, which had been due to depart on 12 October, was postponed for 24 hours, and by the time we began investigating the following day the passengers had left Heathrow and were on route to Angola’s capital, Luanda. Raising our interest in the story via Twitter, we asked for help in locating someone who could visit the airport to interview disembarking passengers.

A freelance did just that, and managed to speak to one who said he had seen three security guards forcibly restrain Mubenga in his seat. We instantly shared that breakthrough, in the hope that it would encourage more passengers to come forward. At the same time we were publishing what we knew about the case, while being candidly open about what we did not know.

Hence the very article, published before any passengers had been tracked down, stated: “There was no reliable information about what led to the man’s death of how he became unwell.” It added, perhaps controversially: “In the past, the Home Office’s deportation policy has proved highly controversial.”

The tone was necessarily speculative, and designed to encourage witnesses to come forward. So too were the tweets. “Man dies on Angolan flight as UK tries to deport him. This story could be v big,” said one.

This articles and tweets, contained relevant search-able terms – such as the flight number – so that they could serve as online magnets, easily discoverable for any passengers with important information and access to the internet. Another tweet said: “Please contact me if you were on BA flight 77 to Angola – or know the man in this story.”

One reply came from Twitter user @mlgerstmann, a passenger on the flight who felt inappropriate force was used against Mubenga. He had come across the tweet – and then read the article – after basic Google searches. “I was also there on BA77 and the man was begging for help and I now feel so guilty I did nothing,” he tweeted.

Within hours, his shocking account of Mubenga’s death was published alongside several other passengers who had found us via the internet. An interactive graphic of the seating arrangements on the aircraft was created, enabling users to listen to audio clips of the passengers give personal accounts of what they had seen.

How verification was crucial

As with the Tomlinson investigation, verification, something paid journalists do better than their volunteer counterparts, was crucial. The fact the passengers had disseminated to remote parts of Africa – @mlgerstmann was on an oil-rig – explains why the only way to contact them was through an open, Twitter-driven investigation.

But this methodology also poses problems for authenticating the validity of sources. Journalists are increasingly finding that a danger inherent in opening up the reporting process is that they become more susceptible to attempts to mislead or hoax. This is particularly the case with live-blogs which need regular updates, require authors to make split-second decisions about the reliability of information and take care to caveat material when there are questions.

For journalists with more time, it is incumbent, therefore, to apply an equal if not more rigorous standard of proof when investigating in the open. In the Tomlinson case, when sources were encountered through the internet it was mostly possible to arrange meetings in person. That was not possible when investigating Mubenga, where there was an attempt by a bogus passenger to supply us false information.

In lieu of face to face meetings, we were able to use other means, such as asking prospective sources to send us copies of their airline tickets, to verify their accounts. What the investigations into the deaths of both Tomlinson and Mubenga show is that journalists don’t always need to investigate into the dark. Through sharing what they do know, they are most likely to discover what they don’t.