Aaron Brady, the man who killed Detective Garda Adrian Donohoe, has been released back into prison following his arrest yesterday.
The 29-year-old was detained on suspicion of witness intimidation, perverting the course of justice and the unlawful use of a mobile phone in prison, in connection with his trial.
He was released without charge this morning.
Gardaí say their investigations are ongoing.
Aaron Brady was sentenced to life in prison in October for the murder of Detective Garda Donohoe in 2013.
Brady, from Crossmaglen, Co Armagh, shot Detective Donohoe from close range during an armed robbery outside a credit union in County Louth in 2013.