Dissident republican Colin Duffy arrested over murder of prison officer David Black

High-profile republican questioned over killing as arrests made both sides of border

Northern Ireland's most prominent republican dissident was arrested today for questioning about the murder of David Black, the prison officer shot dead in a motorway ambush on Thursday.

Colin Duffy, 44, who was acquitted in January of the murder of two soldiers, was detained in his home town of Lurgan, Co Armagh, with a 31-year-old man. The two were taken to a specialised interrogation centre near Belfast where suspects can be held for several days without charge.

A third man was arrested in the Irish Republic tonight in connection with the killing. Police said the 29-year-old was being detained at Carrick-on-Shannon Garda station in Co Leitrim.

Crimestoppers is offering a reward of up to £10,000 for information leading to the arrest and conviction of those responsible for the murder.

Mr Black, 52, was shot on Thursday as he drove along the M1 motorway to work at Maghaberry prison. His car was hit several times by shots fired from a vehicle. It then veered into a ditch.

The incident has generated shock and widespread condemnation, and the murder inquiry is continuing alongside a review of security precautions for jail workers. Thirty prison officers died in the Troubles but the last killing took place in 1993. Describing the incident as a dreadful tragedy, David Cameron said: "The Government I lead will do whatever we can to help the police service of Northern Ireland bring the perpetrators to justice."

Mr Black, who was considering taking early retirement after 30 years as a prison officer, was described by a colleague as "one of life's gentlemen, jolly and full of fun". Former prisoners, both republican and loyalist, called a local BBC phone-in programme yesterday to say they remembered him as a particularly helpful and obliging officer.

The Rev Tom Greer, of Molesworth Presbyterian Church, said Mr Black was "a lovely guy to know in so many ways". He added: "David was not a man to make enemies of anyone. He was a guy whose friendship I and others in our church valued greatly."

He said the Black family had appealed for no retaliation from any quarter, and had asked him to convey that "sadness and grief in another home will achieve nothing".

Sue McAllister, director-general of the Prison Service in Northern Ireland, said after meeting Mrs Black: "The family is completely shattered. They are very close. It is a dreadful tragedy."

The wave of condemnation continued with the government in Dublin pledging full support for police in Belfast. Prime Minister Enda Kenny said any assistance would be "immediately forthcoming" and Irish police would co-operate closely in terms of intelligence and information.

The chairman of the Prison Officers' Association, Finlay Spratt, said he was not aware of serving officers being refused weapons for personal protection, but that in other ways "many security measures have been stripped away".

The Northern Ireland Secretary, Theresa Villiers, told the Commons the authorities would continue to "bear down hard on terrorism" and would provide police with appropriate resources. She said cross-border security co-operation had never been better.

Northern Ireland's Deputy First Minister, Martin McGuinness of Sinn Fein, said those who supported dissident republicans must realise they were backing people who were "swimming in a sea of criminality and drugs". He said such groups were "dressing up their criminality in a flag of political convenience".