A mother and two daughters are being questioned by police investigating the loyalist paramilitary murder of solicitor Rosemary Nelson.
The three detained in Camborne, Cornwall, are originally from Portadown, Co Armagh.
It is understood they have been living in England for over a year.
The 51-year-old woman and her two daughters, aged 24 and 26 were held in an arrest operation involving officers from Devon and Cornwall.
Mrs Nelson, 40, a mother-of-three, was killed in a booby trap explosion under her BMW car outside her home in Lurgan, Co Armagh, in March 1999.
The three women were arrested on suspicion of committing serious terrorist offences in Northern Ireland, according to a statement.
Nobody has been charged with Mrs Nelson's murder, but the three arrested were among several people interviewed by the inquiry team headed by Colin Port.