Footage from a lost documentary showing how the IRA trained new recruits in weapons classes will be aired by the BBC.
The programme also includes a bomb attack being planned and carried out in Belfast.
The footage from a lost American documentary about the IRA is revealed in a BBC Northern Ireland series marking the 50th anniversary of the start of the Troubles.
Last week, Spotlight on The Troubles: A Secret History, showed previously unseen footage of Martin McGuinness carrying weapons and taking part in a car bomb attack in Londonderry in 1972.
Additional film emerges in this week's episode, which covers the worst period of violence in the early 1970s.
Reporter Darragh MacIntyre also makes new revelations about the 1975 IRA ceasefire and the changes in the IRA leadership that followed.
The rediscovered footage includes a bomb attack being planned and carried out in Belfast, gunmen in Derry, and weapons classes for IRA recruits.
The pictures were filmed for an American documentary called The Secret Army.
It was based on a book by the same name written by New York academic J Bowyer Bell.
The documentary was filmed in 1972, but disappeared after a few screenings in America.
The BBC Spotlight team tracked down some of the programme makers and they have also made discoveries about the film's disappearance, which will be revealed later in the series.
IRA members allowed the Secret Army crew to film a number of attacks that were carried out without masks.
They included the planning and execution of a May 1972 bomb attack on the Queen's University Sports Hall at Upper Malone, Belfast.
Several people were hurt in the blast.
The documentary also captured IRA attempts to shoot down helicopters in Derry, a Belfast IRA meeting led by Seamus Twomey, who later became the organisation's chief of staff, and the funeral of IRA member Colm Keenan.
The IRA killed over 800 people during the period covered by this week's episode of Spotlight on The Troubles: A Secret History.
The overall death toll in this period (1972-78) climbed tenfold, from just over 200 dead to more than 2,000.
Episode two of Spotlight on the Troubles: A Secret History will be shown on September 17 at 9pm on BBC One Northern Ireland and BBC Four.
Episode One can be viewed now on the BBC iPlayer.
Never-before broadcast footage of the late Martin McGuinness showing children guns and at the preparation of a car bomb has been unearthed for a major new TV documentary.
Ballybofey is a busy market town in Co Donegal with around 5,000 people. It sits on the national road network 20 minutes from the Border. The main Bus Eireann depot for buses coming from all over the country is five minutes across the river in Stranorlar.