Detectives investigating the 1998 Omagh bombing today arrested a 34-year-old man.
He was detained in the South Down area in connection with the Real IRA bomb which claimed 29 lives.
One man is already in custody charged with the murder of the 29 people, but the trial of Sean Hoey, 36, of Molly Road, Jonesborough, Co Armagh is unlikely to go ahead until well into next year.
Relatives of the dead are to meet Prime Minister Tony Blair at Downing Street next week.
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It will be the first time they have met since the atrocity in August 1998.
They are also having a meeting on Thursday with the Northern Ireland Police Ombudsman Nula O'Loan, whose officers are carrying out an investigation into certain aspects of the police investigation.