The 51-year-old was detained in the Irish Republic on Monday over the killing of David Black in a drive-by gun attack on the M1 last year.
A second man, aged 34, remains in custody at Cavan Garda Station.
Mr Black, a father of two, was shot dead as he drove from his home in Cookstown, Co Tyrone, to Maghaberry prison just outside Lisburn, Co Antrim.
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