The Guardian has removed 13 items from its website after an investigation into accusations of fabrication committed by a freelance journalist.

In an editor's note, Guardian US editor Lee Glendinning said the posts were written by Joseph Mayton, a freelance contributor who's written or contributed to more than 60 items for The Guardian. Mayton began contributing in May 2015; The Guardian's website shows his most recent byline from January.

The Guardian's response was first reported Thursday by Sydney Smith, the managing editor of iMediaEthics.

In the note, Glendinning says The Guardian became suspicious after sources claimed they never spoke to Mayton for an article he wrote about them. They then hired fact-checkers to comb through his contributions and found further evidence of fabrication: stories about events the organizers said Mayton didn't attend, missing sources and individuals who denied speaking with the reporter.

Mayton denied wrongdoing in conversations with Guardian editors:

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.

In addition to unpublishing the articles, The Guardian is also reviewing its guidelines for freelance contributions and examining its use of anonymous sources, according to the note.

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.