Mr Frank McBrearty jnr has told the Morris tribunal that gardaí in Donegal wanted his father to die while under arrest in December 1996 during the investigation into the death of a Raphoe cattle-dealer, Mr Richie Barron.
"They didn't care about the McBrearty family, and if my father died in Letterkenny Garda station," Mr McBrearty told the tribunal chairman, Mr Justice Morris. "They did everything in their power to block us from clearing our name."
Mr McBrearty said his father was examined by a consultant and diagnosed with severe blood pressure, at severe risk of heart attack or stroke.
A hospital consultant, Dr Callaghan, had handed in a letter to the station about the Raphoe publican's medical condition.
"It was a copy letter. It was CCd to me," said Det Sgt Sylvie Henry, who faced cross-examination by Mr McBrearty. "The actual letter was addressed to 'Superintendent Letterkenny'."
Det Sgt Henry said he showed the letter to senior officers in Letterkenny station, but didn't realise until later that the custody officer hadn't seen it.
Mr McBrearty also told the tribunal that he had not met Det Sgt Henry on October 14th, 1996, the day Mr Barron died.
"I never spoke to you in my life until this day," he said.
Det Sgt Henry said this was "incredible."
Mr McBrearty also said that he had not made a statement dated October 18th, 1996. He said there were "inconsistencies" in statements that Det Sgt Henry had made about the investigation.
Earlier, Mr Mark McConnell said he found it strange that Det Sgt Henry and a morgue assistant had examined Mr Barron's remains.
"You had two people in here who had no medical training whatsoever, and they were coming to a conclusion as to the cause of death," he told the chairman.
"This could have been the catalyst to all the arrests. It started in the morgue as far as I can see, where the cause of death was looked upon as an assault rather than a hit-and-run."
A former Garda superintendent, Mr Kevin Lennon, said he had a heated discussion when Insp John McGinley told him his handwriting was found in a notebook discovered in the bedroom of a police informer during the investigation. Mr Lennon said he was "incensed" at the accusation.
Det Sgt Henry said he did not remember the discussion, which Mr Lennon recounted to the tribunal.
He said it was possible he had left before the discussion ended to return the notebook to the strong room "because I honestly 100 per cent do not remember that".
Mr Lennon said that after he arrived in Letterkenny in February 1997 he did not interfere with the investigation which resulted in statements against the McBrearty family being withdrawn by a key witness.
The tribunal resumes on Tuesday.