Two people are being questioned in Northern Ireland in connection with the Omagh bombing that killed 29 people more than five years ago.
Police said the man and woman were detained in the Jonesborough area of South Armagh today.
A PSNI spokesmansaid it was part of a larger operation involving police and the British army.
It is understood up to 200 police officers and Army personnel were involved in the arrest operation.
It is still unclear if charges are imminent.
The pair are being interrogated at Gough Barracks, Armagh city, and can be held for up to 48 hours.
"We are delighted things are starting to move forward," said Mr Michael Gallagher, whose 21-year-old son, Adrian, died in the bomb. "It's early days but obviously the police felt they had sufficient evidence which was strong enough to arrest theseindividuals."
It is understood the arrests are linked to one of the cars which is thought have been used at the time the 500lb bomb was transported through the west Tyrone countryside before being abandoned, killing 29 people, one of them a woman pregnant with twins, in August 1998.
One of the misleading bomb warnings which left police ushering hundreds of civilians into the path of the car bomb instead of away from lower Market Street was made from a telephone box in south Armagh.
Only one man, Colm Murphy, a publican from Dundalk, Co Louth, has been jailed in connection with the atrocity. He was sentenced to 14 years in prison in June 2002 for conspiracy to cause an explosion.
Michael McKevitt, the former leader of the "Real IRA", widely blamed for carrying out the bombing, was convicted last month of directing terrorism. He was jailed for 20 years.
However, the Special Criminal Court was at pains to point out that the offences for which he was convicted happened in the years after the Omagh bombing.