British police, the security agency MI5 and the Garda have spent much of the past year in a search, so far fruitless, for the terrorist unit which has been infiltrated into London to carry out bomb attacks on military or high-profile targets.
It is suspected that the Continuity IRA, a group formed in the early 1990s which supports the political stance of the splinter political party Republican Sinn Fein, is the group responsible for the attacks. It is also suspected that some of the people involved have no previous record of republican terrorism.
The Continuity IRA has an unknown number of members in the Republic and has support in the Derry-Donegal area, Fermanagh-south Donegal and Belfast.
The group has issued few public statements, but it is known to be opposed to the political process in the North on the grounds that it underpins partition. It has its origins in the split in Sinn Fein in 1986 when a group led by veteran republicans walked out of the ardfheis, leaving the party under the Northern leadership of Mr Gerry Adams and Mr Martin McGuinness.
The split was precipitated by the ardfheis decision to drop the traditional republican policy of abstaining from taking seats in the Dail.
The Continuity title derives from its belief that it is the true inheritor of the original IRA and the first Dail. It does not recognise the legitimacy of the Republic's Government.
In one of its statements, after the bombing of a family-run hotel in Irvinestown, Co Fermanagh, on February 8th last year, the group said it would continue its campaign "regardless of how British rule in the six occupied counties is remodelled". It then called on Provisional IRA members to hand over their weapons to it.
Referring to calls from Sinn Fein for it to disband, it said: "We wish to state that we treat calls for our disbandment from those nauseating hypocrites in the same contemptuous manner in which their British paymasters have treated their calls for the disbandment of the RUC."
The Continuity IRA normally does not admit responsibility for attacks. But it did issue a statement admitting it was responsible for an attack which severely injured an RUC constable in Castlewellan, Co Down, on November 1st last.
The group is believed to have been responsible for stealing commercial explosives from a mining company in the midlands late last year with the intention of carrying out attacks in Britain.
IT IS also known that the group has been recruiting young people with no history of involvement in republican politics or paramilitary activity. A number of figures in the organisation have been inculcating young recruits with doctrinaire militant republicanism, preaching that the Provisional IRA and Sinn Fein have "sold out" and are now partitionist.
Recruitment is strictly controlled, and this has made it difficult for the Garda to infiltrate it. The group rejects former Provisional IRA members and anybody with a criminal background.
It is short of funds and has been engaged in a campaign of robberies, particularly in Dublin and the north-west. It was behind the abduction of a bank clerk in Donegal on September 18th last and is also believed to have been responsible for the abduction of a Donegal bar-owner at the weekend during another attempted robbery.
It was also responsible for a robbery at a bookmaker's at Bluebell in west Dublin on September 21st last.
The group carried out bomb attacks on commercial targets including two hotels and a disco in Fermanagh during the first Provisional IRA ceasefire in 1994-1995. It carried out four attacks on British military posts in the Derry area in the past year and in January came close to killing 20 soldiers in a mortar attack on the officers' mess of Shackleton Barracks.
On February 20th it left a booby-trap bomb at a Territorial Army base at White City, close to the scene of yesterday morning's explosion in Shepherd's Bush.
It also planted a bomb under Hammersmith Bridge on June 1st last year; another at Acton Underground station on July 19th last; and fired an anti-tank rocket at the British Secret Intelligence Service headquarters at Vauxhall Bridge on September 20th.