Security forces have reportedly identified a suspect behind the leaking of confidential memos from Britain's ambassador to the US.
Sir Kim Darroch resigned last week after extracts from how diplomatic cables were published in the Mail on Sunday, describing Donald Trump's administration as "clumsy and inept".
It comes as fresh details of their content emerged earlier today.
The Met Police's counter-terrorism command launched a criminal investigation on Friday.
The Sunday Times reported earlier that unnamed government sources said a suspect had been identified and the possibility of a foreign state computer hack had been ruled out.
Scotland Yard and the intelligence services believe a civil servant with access to historical Foreign Office files mounted a recent raid to steal the material, according to the newspaper.
“They think they know who did the leaking,” an unnamed source said. "It's now a case of building a case that will stand up in court.
"It was someone with access to historical files. They went in a grabbed a range of material. It was quite crude."
Intelligence officials from GCHQ are also to join the investigation to find the suspect, reports say.
Met assistant commissioner Neil Basu faced widespread criticism after warning media organisations on Friday that they could break the law if further details from the cables were published.
However, the Mail on Sunday published more details from the memos, in which Sir Kim Darroch called Donald Trump's abandonment of the Iran nuclear deal "diplomatic vandalism".
In a telegram to the then foreign secretary Boris Johnson, Sir Kim wrote: "On the substance, the administration is set upon an act of diplomatic vandalism, seemingly for ideological and personality reasons – it was Obama’s deal.”
