Police investigating the murder of Constable Ronan Kerr have been granted a five-day extension to question a 33-year-old man arrested in the Omagh area on Friday.
Constable Kerr was killed by a bomb under his car outside his Omagh home.
Two other men are also being held over the murder of the PSNI officer.
Brian Carron, 26, from the Dungannon area, was arrested in Scotland on Tuesday and a 40-year-old man was detained near Omagh on Thursday.
The 40-year-old man was arrested when a van was stopped near the Beragh turn-off on the Ballygawley line in County Tyrone.
Both men are being held at Antrim police station.
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.
Mr Kerr's funeral witnessed unprecedented scenes - with police officers and members of the Gaelic Athletic Association uniting in his memory, as well as politicians from across all the main Northern Ireland parties.