At the Guardian, we cherish the trust our readers place in us to provide an accurate and vivid account of the world. That’s why we acted immediately to investigate when sources claimed that they had not spoken with the writer of the piece they were quoted in.

The article in question, from February, was by a freelance journalist, Joseph Mayton, who began writing opinion pieces for the Guardian in London in 2009, while based in Egypt. He contributed several opinion pieces before starting to write occasional US news stories, on a freelance basis, in May 2015 from California. These stories ranged from coverage of wildfires to issues related to marijuana farms, urban vineyards and whale deaths on the coast.

When Mayton was unable to provide convincing evidence that the interviews in question in the February article had taken place, we hired an independent fact-checker to investigate all of his prior work, which comprised 37 single-byline articles published between 2015 and 2016, seven shared byline stories from the same period, and 20 opinion pieces written from 2009 to 2015.

In an investigation that included approximately 50 interviews, our fact-checker found articles that contained likely or confirmed fabrication, including stories about two events that organizers said he didn’t attend. Dozens of sources could not be found – either they had no online presence or they were anonymous and could not be substantiated – and several people quoted in Mayton’s articles either denied speaking with him or giving the quotes attributed to them.

Our editors met with Mayton twice in person and emailed him dozens of times, giving him more than a month from the time the first allegations were presented to him to provide notes, phone records, contact information and other evidence. All evidence he provided has been taken into account, but he was unable or unwilling to provide information on most sources.

Mayton has denied any fabrication, and did not provide any on-the-record comment about the findings of our investigation.

In light of the extent of the fabrication and the uncertainty surrounding many of the articles, we are removing 12 of the news stories, and one opinion piece from the Guardian website. In the articles that remain, quotes and information that could not be verified have been removed, and we have published footnotes on each article page to outline this. There were other stories which proved accurate, with no corrections needed, and have been left as is.

How did this happen? We receive thousands of ideas and pitches from freelancers every year, and we commission large volumes of writing from those not on staff. All freelancers are asked to abide by our editorial code and we take care to make sure that their reporting and writing is of the quality we expect, and you, our readers, deserve.

Mayton wrote only sporadically for the Guardian – he was an infrequent but long term contributor and many different editors in the organization have dealt with his work across opinion, news, arts, sports and features. At first, the quality of his writing met our standards, and thus a trusted working relationship was established.

It is incredibly difficult to detect manufactured quotes – the voices of people on the street who cannot later be verified, for example – which can go unquestioned without a reason to draw the attention of an editor to query them.

During the course of this investigation we have questioned ourselves at length over what we could have done differently to prevent this from happening and have come to two firm conclusions as a result.

First, we need to know more about the freelancers we work with before they start writing for us regularly. These are often people we haven’t met, but we go on to place an inherent degree of trust that the work they are filing to us is true. We are currently reviewing our processes in order to improve due diligence in relation to regular contributors.

Second, we need to take a closer look at occasions where individuals are not named in a story for no strong reason. We need to question the use of anonymous sourcing in any story; a policy we hold, but have not enforced strictly enough. There are occasions where it is of course necessary, such as in sensitive national security reporting, but these stories were not among them.

We want to apologize to those people whose words were misrepresented or falsified. We also want to say sorry to you, our readers, for the errors that have been made here, and hope that it has not compromised the trust you place in the Guardian. We assure you we will do better.