The find provides more evidence that not all of the Provisional IRA's weapons were decommissioned.
BBC: Worn and rusting rocket launcher still capable of killing
The last RUC police officers to be killed by the Provisional IRA were shot dead in Lurgan.
Mr McQuillan said the investigation had taken the police "into the very heart of the Provisional IRA".
Four Provisional IRA members were later convicted of killing the detective, but have since been freed from jail.
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During the troubles, the Provisional IRA frequently mounted attacks at those times to signal that they remained a threat.
"If this murder was the work of the Provisional IRA then very serious consequences will flow from that, " he said.
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The Provisional IRA used a range of mortars, including multiple launchers and much larger so-called "barrack busters" devices during the troubles.
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The Real IRA said he was a member of its organisation and his family has accused the Provisional IRA of the killing.
Following the double murder, the Provisional IRA claimed responsibility, stating that it was a reprisal for recent attacks by Loyalists on Republicans.
On Tuesday, the inquiry's legal team revealed that it had had a face-to-face meeting with three former members of the Provisional IRA in recent weeks.
They were shot dead by the Provisional IRA (PIRA) in an ambush in south Armagh, shortly after they left a meeting in Dundalk garda station.
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Dozens of Russian-made rocket launchers were brought into Northern Ireland by the Provisional IRA in a series of consignments of weapons from Libya during the 1980s.
BBC: Worn and rusting rocket launcher still capable of killing
Sinn Fein, often shunned by maintream politicians because of its links to the Provisional IRA, had four seats, as many as the Greens, the junior coalition partner, in the outgoing parliament.
We have a long, deep folk memory not only of the Blitz which, of course, was the most colossal disaster in London--over 40, 000 died in that--but also the provisional IRA bombing campaign in the '70s and into the '80s.
The question is, have dissidents had this rocket launcher in their arsenal for a long time, or was it acquired much more recently when a former member of the Provisional IRA with access to an old arms dump decided to join dissident ranks?
BBC: Worn and rusting rocket launcher still capable of killing
It says it "believes that relations would improve further" with an apology from the current Irish government for "the role played by the Irish government of the day in the emergence of the Provisional IRA and the roles of past governments regarding the pursuit of terrorists".
The Lagan Valley MP proposed that the party should "reaffirm its commitment to re-enter the executive upon receiving a report from the International Disarmament Commission that the Provisional IRA have commenced credible and verifiable decommissioning of their illegal weapons and explosives, leading to complete disarmament within an agreed timetable".
Some of the dissidents seem to have deliberately eschewed the business of talking to go-betweens and forming political front groups, as they regard this as the first step on the road to being seduced away from violence, just like the Provisional and Official IRA before them.
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