Gardaí investigating the source of a leak which culminated in Trevor Sargent's resignation last week have acquired the phone records of the reporter who broke the story and believe that his source is a serving garda.

The Sunday Tribune understands that detectives have carried out a preliminary analysis on the mobile phone traffic of Evening Herald reporter Conor Feehan and have identified several conversations he had with a garda who could have had access to the correspondence sent by Sargent requesting that a criminal charge against a constituent be dropped.

Sources say that the possibility that a political source leaked the documents has now been all but discounted.

Sources say that gardaí will this week attempt to interview Feehan and several other Evening Herald staff about the source of the story.

It is understood that the newspaper will refuse to cooperate with the investigation which would leave gardaí with little choice but to arrest the journalist.

As part of the investigation – headed by two of the country's most senior gardaí – all officers involved in the investigation into Sargent's constituent will be interviewed, including the garda who has been in touch with Feehan's phone.

The officer will be asked to account for any conversations with the journalist and if they cannot offer a reasonable explanation they could also be detained for a breach of the official secrets act.

Chief superintendent Michael Finn and detective superintendent John McMahon have been appointed to investigate the leaking of documents detailing the unlawful contact Sargent made with gardaí using official Department of Agriculture headed paper.

Sargent, who resigned once the story broke last Tuesday, had written to gardaí in Balbriggan requesting that a pending prosecution against one of his constituents for a public order offence, be dropped.

Copies of the correspondence ended up in possession of the Herald and only the gardaí, Sargent and the constituent in question, had copies.

It is understood that garda commissioner Fachtna Murphy and justice minister Dermot Ahern are determined to identify the person who leaked the documents.

Evening Herald deputy editor Ian Mallon says that the newspaper will not be cooperating with the garda investigation and will not be revealing the source of its story.

Mallon said: "Only in Ireland can a politician break the law, and the whistleblower suffers.

"This sort of intimidation will not prevent us from pursuing stories that are clearly in the public interest. Our highlighting of the illegal and grubby dealings of a Green junior minister seems to have provoked more ire amongst senior gardaí, than the unlawful behaviour of Sargent."

The man that Trevor Sargent was representing, Dominic McGowan, was subsequently fined €500 for threatening and abusive behaviour after gardaí proceeded with the case.