Constable Allen Moore had been held by police and had his personal protection firearm confiscated after firing shots into the air, the inquest was told. But some hours after being released from police custody, in February last year, he went to the Sinn Fein office in the Falls Road and used a shotgun to kill three Catholic men.
The proceedings are expected to last three weeks and are scheduled to include evidence from 98 witnesses, the largest number called to an inquest in Northern Ireland.
The inquest is into the deaths of 24- year-old Constable Moore and the three men he killed; Patrick Loughran, 61, Pat McBride, 40, and Michael O'Dwyer, 21. The first two were members of Sinn Fein while Mr O'Dwyer had called at the Sinn Fein office to register a complaint.
The Belfast coroner, John Leckey, told the jury that Constable Moore had been detained in Comber, Co Down, the night before the shootings, after he had fired shots over the grave of a colleague who had met a violent death not related to the troubles. He was detained as he was reversing his car at speed through the town. He had accompanied police to a station where his blood was tested and found to contain more than two and a half times the legal alcohol limit for driving.
The coroner said that after being interviewed he had spent the night at the home of a colleague, but had failed to turn up for an RUC medical the next morning. Frantic efforts had been made to trace him, as concern mounted for the safety of the officer and others. That afternoon the constable had gained entry to the Sinn Fein office by misrepresenting who he was, and shot dead three innocent people with a shotgun hidden in a suit-carrier.
Later his body was found 15 miles away on the edge of Lough Neagh. He had been shot through the head and the shotgun was beside him. A police officer who had detained Constable Moore in Comber told the inquest that he had been upset. His police issued weapon was taken from him and he was escorted to a police station.
A doctor who examined the constable in the station said he had said to him: 'Look doc, you're here to see if I'm mad. Well I'm not mad, I'm just drunk.' The doctor said he gave the impression of suffering from an unusual degree of grief and depression, but said he had not detected anything that indicated psychiatric illness.
The hearing continues today.