One prison officer has been killed and another seriously injured during the mass break-out of Republican inmates from the Maze jail near Lisburn.
Ten of the prisoners - who were all from Block H7 - were recaptured in the first few hours, but the remainder are still on the run.
Security forces mounted the biggest search operation Northern Ireland has ever seen within minutes of the escape.
The prisoners used smuggled guns and knives to overpower staff before hijacking a food lorry which they used to drive to the main gate.
Stabbed
An officer unsuccessfully attempted to block the entrance to the County Antrim prison with his own car.
But the armed Republicans responded with violence and during the course of the escape one prison officer was stabbed to death. Another was shot in the head and is in hospital.
Several of the men who escaped hijacked private cars when they were outside the prison.
Soldiers and police have now sealed the area and set up checkpoints on all roads within five miles of the prison.
Reports say four were discovered in a river close to the prison, underwater and breathing through reeds.
The escape is the largest in Northern Ireland's history and the biggest at the "H-blocks" since 1974 when 30 Republicans tunnelled out of the complex.
In Context:
The breakout was masterminded by Dermot Finucane. Nineteen prisoners were recaptured within a few days of the breakout, but the rest got away.
By 1992 five more had been caught and three killed in ambushes.
Some of those involved in the escape have never been recaptured and others were given amnesty as part of the Northern Ireland peace process.
The closure of the Maze prison was written into the Good Friday Agreement and the last inmates were transferred in September 2000.