Gerry Ryan with long-time colleague Siobhan Hough in the 2fm radio studio

GardaÍ have seconded six of the country's most experienced murder detectives to investigate claims made in the Sunday Independent that corrupt gardaí protected Gerry Ryan and other high-profile people who were drug users.

Last week's newspaper claimed that "a small number of corrupt gardaí in Dublin have protected rich and public figures with drug habits, including Gerry Ryan".

Following the publication of the story gardaí announced that the allegations would be fully investigated and Detective Superintendent John McMahon was appointed to head the probe. John McMahon has a team of at least five National Bureau of Criminal Investigations (NBCI) detectives working with him. They primarily investigate gangland murders and will be working on this investigation for upwards of two months.

McMahon arrived at the Sunday Independent offices last Tuesday with a letter requesting that journalist Jim Cusack and editor Aengus Fanning consent to be interviewed. It is understood the pair will talk to gardaí in the New Year. However, they will not cooperate with the investigation by naming sources and are standing over the story.

It is likely that Cusack's phone records have been accessed by detectives in a bid to determine if he spoke to any gardaí before publishing the story. This is a trend increasingly adopted by garda management to deter officers from talking to the media.

Senior gardaí believe McMahon's report to the garda commissioner will state that there is no evidence to back up the newspaper's claims but sources privately acknowledge that the story referred to a now retired senior officer who has a reputation for being close to several high-profile personalities.

However the sources also add that they do not believe this officer ever prevented colleagues from arresting the man who is believed to have supplied Gerry Ryan with cocaine.

Sources in the Sunday Independent believe the garda investigation was prompted by "over the top" coverage of the newspaper's story on RTé, which has been criticised for effectively ignoring the revelations that Ryan was a cocaine user. They believe the national broadcaster was only too happy to deflect the public attention away from how it was handling the Ryan story.

The head of RTé corporate affairs, Kevin Dawson, last week acknowledged that the broadcaster was wrong to limit its coverage of the fallout from the Ryan cocaine revelations.

As well as announcing the Sunday Independent investigation, gardaí also surprisingly revealed that an investigation into who supplied Gerry Ryan with cocaine was still taking place. This is despite the fact that the investigation had effectively concluded. It is believed that public pressure forced this u-turn.

John McMahon is one of the gardaí's best and most experienced detectives and is regularly tasked with investigating stories printed in the media.

He was in charge of the investigation into the leaking of a letter to the Evening Herald about attempts by Trevor Sargent to influence a garda investigation earlier this year; he also led the probe into the leaking of an unpublished report into the Dean Lyons affair which led to the arrest of this reporter in 2007.

Neither Jim Cusack nor Aengus Fanning would comment when contacted.

Gareth O'Callaghan, a former RTé colleague of the deceased DJ, told the Sunday Independent he intended to inform gardaí of the identity of Gerry Ryan's drug dealer. However, sources have confirmed that in his interview with detectives last week, the 4fm DJ did not supply them with any names of people who could have supplied Ryan with cocaine.

"He gave us nothing. He talked a good game in the newspapers but had nothing for us," said a source.

While gardaí are aware of the various drug dealers who Ryan could have been buying cocaine from, it is understood that the broadcaster was not buying direct from a dealer but through associates and friends. This will make the reopened garda probe into who supplied him with cocaine difficult. At present, no arrests are imminent.

Detectives also suspect that on the night he died, he only did a "line or two" of cocaine and didn't buy any on the night he died, which again complicates their enquiries.