British military intelligence knew about the existence of a supposedly secret IRA unit called “the Unknowns” months before it started abducting, killing and secretly burying victims who became remembered as “the disappeared”.

A military file recently unearthed from the National Archives in Kew in Surrey reveals that army commanders knew about the Provisionals’ Unknowns unit as early as April 1972.

This means the army had knowledge about this secretive IRA group several months before it started to “disappear” its first victims, including Jean McConville in west Belfast.

McConville’s abduction, murder and secret burial has become one of the most controversial cases in the Northern Ireland Troubles. This is because the former Belfast IRA commander Brendan “The Dark” Hughes alleged that the former Sinn Féin president Gerry Adams gave the order for the widow to be disappeared rather than have the republican movement admit publicly it killed her.

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Adams has consistently denied he had anything to do with the McConville murder and has also rejected Hughes’ claims that at the time he was a senior IRA leader in Belfast.

The British military document was published on Sunday on thebrokenelbow.com, a website run by a leading expert on the IRA, Ed Moloney.

The new material includes an army report about an armed robbery carried out in north Belfast on 25 April 1972. The raid was intercepted by soldiers who arrested three men and recovered a pistol in Manor Street, in the Oldpark district.

Among those captured by troops was a man referred to in the document originating from army headquarters at Thiepval Barracks as “Vol: the Unknowns”. That is to say, a volunteer in the Provisional IRA and a unit even most Belfast republicans were not supposed to have known about.

Facebook Twitter Pinterest Michael McConville, son of Jean McConville. Photograph: Niall Carson/PA

Moloney told the Guardian this suggested many in the British army had known about the Unknowns from an early stage.

“The revelation that the British military had enough knowledge of the unit to quickly identify one of its members begs some obvious and potentially difficult questions for Whitehall: how and what did the army know about the unit, did they have a source inside the Unknowns and, crucially, what, if anything, did the military know about the IRA’s practice of disappearing people?” he said.

Moloney, the author of A Secret History of the IRA, said he had contacted the man whom the army arrested in the robbery. He was later tried, convicted and sent to the Maze prison at Long Kesh.

Named only as “Gerald” on thebrokenelbow.com, the ex-IRA prisoner confirmed he was once a member of the Unknowns. He said he had been part of the north Belfast unit of the Unknowns, which comprised two cells, the other being in west Belfast.

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The west Belfast cell was led by the late republican Pat McClure, who was in charge of kidnapping suspected informers and ferrying them across the border into the Irish Republic where they were interrogated and sometimes tortured before being shot dead and buried in secret graves. McClure was interned in 1973 and later died of lung cancer.

Another element of the British Army document on the arrest of one of the Unknowns in the Manor Street robbery was an order to pass on the information about “Gerald” to the military’s psychological warfare branch in army headquarters in Northern Ireland.

This propaganda branch, known as the Information Policy Unit (IPU), was tasked with spreading information including fake news to undermine the credibility of insurgent groups such as the IRA.

Moloney said there was no current evidence to suggest this army propaganda unit used its authentic knowledge of the Unknowns to smear the IRA with involvement in disappearing victims such as McConville.

“It may be that someone in the military decided it was better to keep quiet about the Unknowns,” Moloney said.

The remains of at least three of the 16 disappeared have not been located. The missing include one of the first to be killed and buried in secret, Joe Lynskey, a former monk turned IRA member who was disappeared in August 1972. Searches began again last month at a bog in County Meath, in the Irish Republic.