The prime suspect in a deadly 1982 IRA bombing whose trial collapsed appeared in a Dublin court on Tuesday facing extradition for the murder of two British soldiers in a separate attack.
On Tuesday Downey appeared for the first time under a European arrest warrant at Dublin's Criminal Courts of Justice.
If prosecutors are successful the 66-year-old will stand trial in Northern Ireland for the 1972 murders of two members of the British Army's Ulster Defence Regiment who were killed when a car they were inspecting blew up.
The fresh prosecution has reignited debate over litigation of historic crimes on both sides during The Troubles -- a period when republicans and unionists waged a bloody battle over the status of the UK province of Northern Ireland.
The four soldiers of the British cavalry where killed alongside seven horses when a nail bomb exploded in a car as they made their way from barracks to the iconic changing of the guard ceremony at Buckingham Palace.
But in 2014 Downey walked free after it emerged that prosecutors had assured him in an amnesty letter that he would not face trial -- despite there being a warrant outstanding for his arrest.
The amnesty appeared in one of 228 "comfort letters" sent out to militants as part of negotiations to preserve the peace brokered in Northern Ireland's 1998 Good Friday Agreement.
Families of the Hyde Park victims are currently taking civil action against Downey.
"We are pleased to see that the judicial process is finally shining a light on (the 1972 attack), and on the long-standing allegations surrounding Mr Downey," said Mark Tipper -- whose brother Simon died in the London attack -- in a statement to AFP.
Pearse Doherty of Northern Ireland's republican Sinn Fein party objected to the arrest.
"John is a staunch supporter of the peace process and has been for many, many decades," he told reporters.
"It's a matter of public record that John has been given assurance by the British authorities that he was not wanted for any offences." In addition to the double murder Downey faces charges of "aiding and abetting an explosion". A bail hearing will take place on Thursday.
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