Two men have been arrested in connection with the murder of prison officer David Black.
Mr Black, 52, died after being shot on the M1 motorway in Co Armagh while driving to work in Maghaberry prison earlier this month.
The men, aged 34 and 42, were detained in Coalisland, Co Tyrone, and are being questioned in Antrim.
A dissident republican group styling itself as the "IRA" claimed responsibility for the murder.
Four men have already been arrested and released in connection with the shooting.