Theresa Villiers MP said the Police Service of Northern Ireland (PSNI) was conducting a "meticulous investigation" into the killing of prison officer David Black.
The Northern Ireland Secretary told the House of Commons that officers investigating his murder "will not rest until the perpetrators have been brought to justice and put behind bars, where they belong."
Mr Black, 52, a married father-of-two, was shot several times from a car that pulled up alongside his on the M1 near Lurgan, Co Armagh, as he drove to work at Maghaberry jail yesterday morning.