Ms Karen McGlinchey told the Morris tribunal that Det Garda Noel McMahon assaulted her in her home and threatened the life of her sister.
Ms McGlinchey, sister of key witness Ms Adrienne McGlinchey, said in May 1999 Det Garda McMahon and another garda had come to her home in Letterkenny, Co Donegal, as she was a witness in an unrelated incident. Det Garda McMahon indicated that he wanted to speak to her privately and they went into the study.
She said Det Garda McMahon told her he was having trouble with his wife, Mrs Sheenagh McMahon, that she was making a lot of trouble and talking. Det Garda McMahon had said his wife had taken "stuff" out of the house. Ms McGlinchey asked him if it was drugs and he replied it was guns and explosives.
She asked where he had got the explosives and he replied: "Adrienne". Ms McGlinchey said she nearly collapsed and could not take it in. Det Garda McMahon told her that he had stopped her sister years ago in Buncrana and that she was transporting it in the boot of the car.
Ms McGlinchey said she did not believe it and started to lose her temper. They were shouting.
"I was really, really angry about it. I had my back to the wall, and the next thing he just pushed me up against the wall. First of all he said that if everybody stayed quiet it would be OK, that he liked Adrienne. He said he would go to jail before her," she said.
Ms McGlinchey said it was not making any sense to her.
"He pushed me up against the wall and he put his arm across my chest. He put his face right up to mine and I could smell the drink off his breath and he actually had turned into this vicious - I can hardly explain. . .," she said.
Det Garda McMahon had said he was going to keep her sister quiet. When she asked what he meant he had said that nobody would hear her sister six foot under.
Ms McGlinchey said she told Det Garda McMahon that she was having none of it and that she had written an affidavit and would have it resurrected in the Department of Justice. He was saying to her not to do anything rash and she told him to get out. Det Garda McMahon and Det Garda Matthew Tolan, who had been in the kitchen, then left.
Ms McGlinchey said after it she was shaking, very frightened and roaring crying. About 10 minutes later, a friend of hers, a journalist, Mr Sean Doherty of Highland Radio, whom she was to have lunch with, arrived and she told him and her mother what happened.
About five minutes after, Det Garda McMahon rang her and told her not to do anything rash, just keep quiet. She had held up the phone so Mr Doherty could hear. She made a complaint about six months later.
Mr Brian Murphy SC, for Det Garda McMahon, said his client was denying he assaulted her and the allegations.
Earlier, Garda Thomas Rattigan told the tribunal he had found bomb-making paraphernalia in Ms Adrienne McGlinchey's flat in Buncrana. He rang Det Garda McMahon, who was off duty, and did not contact the Garda station. He said he had been off-duty and visiting in the flat downstairs in March 1994 when the landlord, Mr John Mackey, called him up. In the bathroom, underneath the bath, there was a steel tubular item and a hundredweight bag of fertiliser.
Asked if he had rung so that Det Garda McMahon could take credit for the find, Garda Rattigan agreed. At the time, he was doing his duty to the best of his ability. In hindsight, he should have returned to the scene and preserved it and contacted the Garda station as well as Det Garda McMahon.