Five men have been arrested in connection with the murder of PSNI Constable Ronan Kerr in April this year.
The arrests were made during police searches in Coalisland, Co Tyrone, and in Bellaghy, Toomebridge and Ballyronan in Co Derry.
A 22-year-old man was arrested in Coalisland. Two men, aged 27 and 46, were arrested in Toome. A 37-year-old was arrested in Bellaghy and the man arrested in Ballyronan is aged 33.
The search and arrest operation focused on several locations around Toomebridge in Co Antrim.
Two homes in the town have been sealed off and are being searched by detectives from the PSNI's Serious Crime Branch.
Outside the town, a pub and restaurant has been cordoned off and a security screen erected as forensic experts and police carry out a detailed examination of the premises.
Similar operations are also being carried out around Coalisland and near Bellaghy.
A number of items were also seized during the operation, including clothing, mobile phones computers and vehicles.
Mr Kerr was a 25-year-old Catholic recruit who had just completed his initial training in the PSNI.
He was killed in April by a booby trap bomb, which exploded under his car in Omagh, Co Tyrone.
The attack was carried out by dissident republicans and the latest operation in the investigation involved 200 PSNI officers and staff.
Detective Superintendent Raymond Murray, said: 'Today's activities are the latest stage in the ongoing investigation into Ronan’s murder and related incidents.
'Our determination to pursue every possible line of inquiry remains undiminished.
'The investigation is progressing and police have been encouraged by the support and assistance which has been forthcoming from across the community.
'During this search stage, we would ask for people’s continued patience and co-operation as we work to complete our inquiries to maximum effect, as expeditiously as possible and with the minimum amount of disruption to community life.'
