Two men are still being questioned by police in connection with the murder of the PSNI officer Ronan Kerr.
Brian Carron, 26, from the Dungannon area, was arrested by police in Scotland on Tuesday and a 40-year-old man was detained outside Omagh on Thursday.
Both men are being held at Antrim police station.
Constable Kerr, 25, was killed when a device exploded as he drove his car from his home in Omagh on Saturday.
Police believe the bomb had already been planted under the car while runners taking part in a half-marathon ran past it.
Police said the bomb could have killed or maimed anyone in the area.
The 40-year-old man was arrested when a van was stopped near the Beragh turn-off on the Ballygawley line in County Tyrone on Thursday.
An apartment in north Dublin was searched on Thursday in connection with the murder.
The search was carried out by members of the Garda (Irish police) Special Detective Unit at the request of the Police Service of Northern Ireland.
Weapons and explosives, including four Kalashnikov rifles were found in the Coalisland area of County Tyrone on Tuesday evening by police investigating the police officer's murder.