The Metropolitan police are continuing their criminal investigation into the leak of diplomatic cables to the Mail on Sunday that led to the downfall of the British ambassador to Washington DC, prompting renewed speculation on how they ended up in the hands of the national newspaper.

Neil Basu, one of the Met’s four assistant commissioners, told an audience in central London that the embarrassing private correspondence had been passed around other outlets before they appeared in the Mail on Sunday in July.

“I certainly now know now that other journalists had been offered the story and refused to print it,” said Basu, raising questions about how the highly confidential material reached the Sunday paper, in a potential breach of the Official Secrets Act.

Kim Darroch quit after it was revealed he described the administration of the US president, Donald Trump, as “inept” and “uniquely dysfunctional”. The leaked material appeared on the front page of the Mail on Sunday, credited to the political journalist Isabel Oakeshott, who has close links to the Brexit party.

Later, the Mail on Sunday claimed the real reporter was Steven Edginton, a 19-year-old who served as the Brexit party’s digital campaign chief and had, the paper said, obtained the documents after a lengthy journalistic investigation into the civil service.

He said he did not want a byline because it would give the impression that the Brexit party was involved in the leak, so decided to pass it on to Oakeshott.

Basu defended his decision to warn against publication of further details from the diplomatic memos, saying it was “the right call to try to intervene in a potential criminal act”.

Kim Darroch quit after it was revealed he described the administration of the US president as ‘inept’. Photograph: Niall Carson/PA

The police officer, who was widely criticised for a perceived attempt to limit freedom of the press, told the Society of Editors’ annual conference: “I do regret the impact of my statement, not because of its effect on me, but because it risked damaging the relationship between policing and the media.

“I am constrained by what more I can say, and in an election period I can’t enter into a debate about the reform of the Official Secrets Act.”

Basu, the UK counter-terrorism chief, has previously warned about the radicalisation of the far right by mainstream media reporting, including decisions to publish terrorist manifestos or include livestreamed footage on websites.

This has prompted him to ask the Royal United Services Institute thinktank to investigate the links between journalism and terrorism, with initial conclusions suggesting that “how journalists frame their reports and the language they choose can have an impact” and there are steps which can be followed to reduce the risk of copycat attacks.

Basu said early findings suggested “significant empirical evidence that media reporting can be the tipping point to radicalisation, recruitment, or carrying out a violent act, through social contagion or so-called mimetic theory”.

As a result, he floated the idea of a voluntary code of conduct for reporting terror offences, similar to the guidance produced for coverages of suicides by the Samaritans, which had proved to be “enduring, mutually beneficial, and practical”.

But he also said he would not want the media to stop using images of terrorists in their reporting. “It is important the public understand who these people are and what they’re about and let us understand how pathetic most of it is,” Basu said. “What I don’t like is that the images are constantly repeated for all eternity.”