Auditor-General Shauket Fakie's hunt for government employees or contractors who leaked details of the arms deal investigation intensified this week with confirmation that former judge Willem Heath was to be interviewed on Friday.

Fakie confirmed he was overseeing a probe into who had leaked  information to M&G writer Paul Kirk for an article published on January 11. The article stated that information available to investigators - on gifts received by a senior naval officer or his relatives from a winning tenderer - had not made it into the final report on the R50 billion-plus arms deal.

Fakie also confirmed a separate probe under his authority, into the leak of confidential "working papers" to an outside party whom he did not specify, but who was "not from the media". In this case members of the special investigating unit, formerly headed by Heath, were under the spotlight.

The Freedom of Expression Institute (FXI) this week branded  Fakie's probe into the leak to M&G a "serious blow to transparency in government". Fakie, however, denied he had embark on a witch-hunt and said: "I am not opposed to transparency and openness at all ... But this probe relates to information [contained in] working papers and classified information."

Members of Fakie's own office have pointed out that information was hardly kept secure - having been stored for months in a pantry.

Fakie said he had a duty to ensure his office's code of conduct was upheld, and confirmed that 25 people - contractors and employees of his office and the two other agencies that made up the joint investigative team into the arms deal, the offices of the public protector and the national director of public prosecutions - had been identified as potential leaks. All 25, whom he said included himself, had agreed to sign affidavits.

Signatories are asked to specify in the affidavits any contact they had with Kirk before January 11 and also whether they object being subjected to lie detector tests and having their office and private phone records examined.

Signatories are also asked to betray anyone they think "may have communicated with the media" - which one arms investigator this week described as "distasteful".

Fakie said the probe, which would have "minimal" cost implications for his office, was being conducted by one staff member and "an external person". He would not be drawn on whether the police, an intelligence agency or any private agency was assisting.

Heath's representative Alan Phillips told the M&G that the former head of the unit had approached Fakie to ask whether he was under investigation for the leaks after he heard rumours he was suspected of leaking information. Heath and the unit he formerly led did a preliminary investigation into the arms deal, but were later excluded from the joint investigation.

Phillips said Fakie had assured Heath he was not a suspect. However, the M&G has been informed that investigators will interview Heath.

On joining the office of the auditor general, staff are required to sign oaths of secrecy. The same oath is signed by contractors, meaning that anyone caught having leaked information could be criminally prosecuted.

FXI communications officer Scotch Tagwireyi said: "The action ... has implications on the free flow of information, which is a cornerstone of democracy."

With acknowledgements to Paul Kirk, Stefaans Brümmer and the Mail & Guardian.