MI5 has put an appeal on its website over the fatal shooting of two soldiers and a policeman in Northern Ireland.
Sappers Mark Quinsey and Patrick Azimkar were shot by the Real IRA in Antrim on Saturday.
Constable Stephen Paul Carroll was gunned down by the Continuity IRA in Craigavon, County Armagh, on Monday.
It is thought the only other time the security service has done this was after the attempt to blow up the Tiger Tiger nightclub in London in 2007.
A 37-year-old man and a 17-year-old youth have been arrested in connection with the murder of Constable Stephen Carroll, 48, in County Armagh on Monday.
On Thursday, detectives were given five more days to question the pair
The Continuity IRA said it shot Constable Carroll as he answered a call for help in Craigavon.