A prison officer has been killed by dissident republicans in an attack on the M1 motorway between Lurgan and Portadown, County Armagh, police have said.
One unconfirmed report claimed a gunman opened up when another car pulled up alongside.
It is understood the officer's car careered off the road when he was fired on and he died later.
The man is the 30th prison officer killed in Northern Ireland since 1974.
Sinn Fein Assembly member John O'Dowd said the shooting was "totally wrong".
"There is no justification for continued conflict in this society," he said.
Mr O'Dowd insisted the extremists would not be allowed to take the region backwards.
"We, as a society, have decided we want to move beyond them. The road they are on is not going to achieve anything."
Paul Girvan, a Democratic Unionist Party (DUP) member of the Northern Ireland Assembly, said the man was on his way to begin duty at the top-security Maghaberry Prison, near Lisburn, Co Antrim, where dissident republicans have been involved in long-running protests against jail conditions.
Jeffrey Donaldson, MP for Lagan Valley, said: "This is a terrible tragedy. Once again, a small minority are trying to drag us back to the dark days."
A car suspected of being linked to the attack was later found burnt-out at Lurgan, where dissident republicans have considerable support.
The shooting happened at around 7.30am, at the same time as a major security alert further along the motorway at a shopping centre at Sprucefield, near Lisburn, where bomb disposal experts were called in to check a car.
It is believed this vehicle might have been linked in some way to the shooting, according to some sources.
Yesterday Northern Ireland Secretary Theresa Villiers warned that the threat from dissident republicans in Northern Ireland remained severe.
That came a week after the threat of attack in Great Britain was downgraded to moderate.
The dissident protests in Maghaberry have taken place at a time when the NI Prison Service is undergoing a turbulent programme of reform.
Today's incident represents a major blow for recently-appointed director general Sue McAllister, 51, from South Yorkshire, who took over the reins of the organisation earlier this summer.