Gardaí are outraged after GSOC opened an investigation into a fourth garda who was a colleague of the sergeant who took his own life last week.
The garda was informed he is now being investigated in relation to a fatal road traffic accident in Ballyshannon, Co Donegal, last January.
This was the same incident which led the late Sgt Michael Galvin to believe he may be charged with involuntary manslaughter.
Two other gardaí have already been cleared - and Sgt Galvin had also been cleared, but had not been told before he took his own life a week ago.
Sources say the timing of this investigation is "crass, outrageous and insensitive". One said: "You couldn't make this up. This whole case must now be taken out of the hands of GSOC.
"The idea that they should consider a fourth investigation when the station is still in mourning is unbelievable. Everyone is extremely angry."
The Association of Garda Sergeants and Inspectors (AGSI) said that the GSOC investigation was a factor in the death of Garda Sergeant Galvin.
Sources have revealed that GSOC officials undertook a cautioned interview with Sgt Galvin in the company of his solicitor on May 20.
The sergeant told friends that he could not take the pressure of the investigation, which had left him feeling like a criminal.
An investigation was launched when an apparent discrepancy arose between Sgt Galvin's account of what happened and the CCTV footage from the street at the time.
In a letter to his family, Sgt Galvin said he could not take the pressure any longer.
Earlier this week, a spokesperson for GSOC confirmed to the Irish Independent that it had cleared the officer of any wrongdoing five days before his death.
However, in an interview on RTÉ yesterday morning, Ombudsman Commissioner Kieran Fitzgerald said Sgt Galvin had been cleared on May 27 .
He said: "Only on the day before his death did we finally conclude what our finding would be."
Mr Fitzgerald also said that GSOC would continue to conduct an investigation into the officer's death despite calls from his colleagues and widow that they should not be involved.
He added that GSOC would commission a peer review of the inquiry.
It is understood it will be carried out by the Police Ombudsman of Northern Ireland.
Garda Commissioner Nóirín O'Sullivan will travel to Ballyshannon tomorrow to meet with colleagues of the late Gda Sgt Galvin. She has also asked to meet his family.