LURGAN republican Colin Duffy is one of two men arrested in connection with the murder of prison officer David Black near Portadown on Thursday.
Duffy (44) and a 31-year-old man were detained in Lurgan on Friday.
They have been taken to Antrim Serious Crime Suite for questioning in relation to the killing.
Earlier this year Duffy was cleared of murdering two soldiers outside Massereene Army Barracks in Antrim in 2009.
Mr Black (52) was shot on the M1 motorway between Portadown and Lurgan as he drove to work at Maghaberry Prison around 7.30am on Thursday.