The DisclosuresTribunal has heard that Garda Keith Harrison threatened to burn his partner and her children during an argument.
The threat was reported to gardaí by Marisa Simms'sister, Paula McDermott.
The inquiry is investigating contacts between Tusla and the gardaí in relation to Garda Harrison.
Garda Brendan Mahon told the inquiry that MsMcDermott, called to Letterkenny station on 30 September 2013.
Garda Mahon said that Ms McDermott told him that during a verbal argument between Garda Harrison and Ms Simms on the night of 28 September 2013, he shouted at her that he was going to burn her and her children in the house and bury her - writes rte.ie.
Garda Harrison will tell the tribunal that during the argument he told Ms Simms that because of the actions of her family, she would be the person getting burnt, meaning that she would suffer the consequences.
Garda Mahon said today that MsMcDermott seemed visibly upset and was concerned for her sister.
He said he took it seriously but in order to begin an investigation, Ms Simms would have to make a statement.
Counsel for the Tribunal Diarmuid McGuinness said the threat to burn and bury Ms Simms seemed to be unequivocally a threat to kill her.
Garda Mahon did not enter the incident in Pulse as he said that would be a decision for the Sergeant. He filed a report to Sergeant Jim Collins.
Sgt Collins spoke to Paula McDermott on 1 October. She was getting married three days later.
Sgt Collins said that while the threat was serious, he feared that gardaí were being used as a pawn because Paula McDermott was worried that Garda Harrison would turn up uninvited at the wedding and make a scene.
He told the tribunal that he felt the emphasis was on Garda Harrison turning up and causing a problem.
He said different options were given to Paula McDermott in order to facilitate getting a statement from her sister, but Ms McDermott indicated that no statement would be made before the wedding.
Garda Harrison's lawyer Mark Harty SC said that no Garda spoke to Garda Harrison regarding the threat.
Conor Dignam SC for the gardaí said the matter was immediately referred to the Garda Siochana Ombudsman Commission.
In addition, an investigation was begun under Garda regulations, but that was stopped by a successful legal action taken by Garda Harrison which prevented statements being taken.