The decision of the Northern Ireland police chief, Sir Hugh Orde, to deploy undercover British special forces against dissident republicans highlighted the controversial and emotive history of the troops during the Troubles.

Nationalists have repeatedly accused the SAS of conducting a dirty war, including assassinations, during the years of violence. Martin McGuinness, the Sinn Fein Deputy First Minister and former IRA leader, described the move as "stupid and dangerous".

One theory is that the announcement on Friday that the Armed Forces Special Reconnaissance Regiment (SRR) was being drafted back into Northern Ireland to keep watch on suspected dissident republican terrorists may have accelerated the timing of the attack on the army base.

Although the shooting would have required considerable planning, including regular monitoring of visitors in and out of the Massereene Barracks in Antrim, the confirmation by Hugh Orde, the PSNI chief constable, of the arrival of the SRR appears too coincidental to be dismissed.

The unit deployed, the Special Reconnaissance Regiment (SRR), was formed four years ago with the specific aim of targeting international terrorism. However, the core membership is drawn from 1st Battalion, The Parachute Regiment and Signals Regiment and many would have seen previous service in Ulster. Much of the covert-intelligence gathering skills of the SRR have also been honed in the province.

The 400-strong group was the first regiment-sized special forces unit to be formed for nearly 50 years. They have their headquarters, like the SAS, in Hereford and fall under the command of the Director of Special Forces.

Members of the SRR were put on the ground in Northern Ireland after a car bomb weighing 300lb was found at Castlewellan in County Down. Their main function, according to defence sources, was to carry out deep cover surveillance, including electronic eavesdropping, rather than take part in immediate offensive operations. "It wasn't a question of them kicking down doors and shooting people in the middle of the night" insisted a senior officer "but providing some much needed intelligence for the police."

The SRR had served in Iraq and Afghanistan. Members were on duty in Basra taking part in an operation to rescue special forces soldiers seized by Shia militiamen in September 2005. They have also been involved in the freeing of hostage Norman Kember in March 2006 and the so far unsuccessful attempts to free five Britons who were seized from the Finance Ministry in Baghdad in 2007.

In Afghanistan, troops from the SRR deployed to Helmand and Kandahar and are said to have played a vital role in a series of operations, a so-called decapitation campaign, in which Taliban leaders were killed. The unit's main role on those occasions was to gather information on the movement of the targets.

The SRR was also reported to have been involved in the operation which led to the killing of innocent Brazilian Jean Charles de Menezes after the 7/7 bombings in London. Members were said to be providing "technical support" for Scotland Yard anti-terrorist officers.