Newly-discovered documents show the Army knew the bomb which exploded at McGurk's Bar was placed outside it, a solicitor said.
Relatives of the 15 killed when the device exploded in 1971 have applied to attorney general John Larkin QC for a fresh inquest to dismiss rumours an IRA device exploded by mistake inside the crowded Belfast pub in a predominantly nationalist area.
Log sheets provide clear evidence that the military technical officer who examined the scene was convinced from the outset that the explosive had been left in the entranceway, their lawyer Padraig O Muirigh said.
Mr O Muirigh said: "It is now well established that the rumours of an IRA own goal were entirely untrue."
Fifteen people died in the attack, the worst Troubles atrocity prior to the Omagh bomb.
A police ombudsman inquiry said the original police investigation had a clear predisposition to the erroneous IRA "own goal" theory.
The watchdog found loyalist paramilitaries were responsible.
Mr O Muirigh said there remained uncertainty how the dead met their deaths and whether there was collusion between members of the security forces and loyalist paramilitaries.
The Ombudsman's report said there was no collusion involving the RUC.
Mr O Muirigh added: "The recently discovered HQ NI log sheets of the 4th and 5th December 1971, discovered by Ciaran MacAirt of Papertrail, provide clear evidence that the Army Technical Officer who examined the scene was convinced from the outset that the bomb had been placed in the entranceway of the pub."
He said the original inquest, a criminal conviction and the reports of the Historical Enquiries Team and Ombudsman failed to determine the facts or allay rumours and suspicions.
"These allegations are so serious as to require proper investigation to establish the facts and circumstances of these deaths despite the passage of time.
"The appropriate way to investigate these deaths and to provide a comprehensive public account of these deaths is through a fresh inquest."
Mr MacAirt's grandmother, Kathleen Irvine, was one of those murdered.
He said: "We have always known our loved ones were innocent and the British state tried to bury the truth along with our dead relatives.
"A fresh inquest allows us that chance to finally hear the truth told after 45 years campaigning."