The investigation has caused disquiet among Garda ranks, many of whom who have received genuine death threats.
The murder of Garda Tony Golden in Louth two weeks ago has hammered home the real threats faced by Gardaí every day.
Sources said the garda is based in the Leinster area and has been subject of genuine threats in the past. However, he is now under suspicion of posting a death threat to his own home address.
Investigators were able to trace back the purchase of stamps which were placed on the envelope of the threatening letter, according to sources. CCTV footage is understood to show the officer buying the stamps.
The news comes at a time when gardaí are under increasing threat from criminal figures.
Just last month a detective based in Louth – the division where Garda Tony Golden and Garda Adrian Donohoe were shot dead – was warned of a threat to his life from associates of the gang suspected of murdering Garda Donohoe.
Morale is said to be at a particularly low ebb after Garda Golden was shot dead by Adrian Crevan Mackin.
“Promises were made about resources after Garda Donohoe’s murder, but they weren’t delivered on,” said a source.
“There’s a feeling that some people are just going to jack it in now. People are at a very low point.”
An additional 25 uniformed gardaí were sent by Garda Commisioner Nóirín O’Sullivan to Dundalk as a temporary measure since Garda Golden’s death. In May it emerged that gardaí at Carbury Garda Station in Kildare were issued with official Garda Information Messages (GIMs) informing them of credible threats against them.
A gang based in rural north-west Kildare and Offaly were behind the threats, which saw an arson attempt and shots fired at the Garda station and at a Garda car.
A leading member of the gang is thug Damien Galvin (above), who previously told one cop: “I’ll leave your wife a widow.”
In a sinister threat to one officer, he made the shape of a gun with his hands and said: “You’ll remember this day when I’m standing over you with a gun to your head and my c*** in your mouth.”
In May, major criminal Paschal Kelly, originally from Coolock in Dublin, was jailed for threats to kill a Criminal Assets Bureau (CAB) officer, among other offences. Kelly flew into a rage after a CAB officer called to his partner’s Balbriggan home, on April 7, 2011, and he subsequently phoned the officer.
“He stated ‘I will kill you, I will f ***ing kill you’, before hanging up,” the detective told a court hearing.
Kelly was jailed four-and-a-half years.