Police in the North questioning three men over the murder of constable Ronan Kerr (below) fear further attacks on their officers are being planned.
A 33-year-old man was detained in the Omagh area yesterday over the booby trap bomb attack that killed the 25-year-old Catholic officer outside his home in the Co Tyrone town last Saturday.
Police were yesterday also given five more days to question a 26-year-old man arrested in Scotland on Wednesday and re-arrested on Thursday, plus a 40-year-old man arrested near Omagh on Thursday.
The under-car bomb was blamed on dissident republicans who remain violently opposed to the peace process.
Senior police sources have said the public outcry following the young officer's death has had no influence on the mindset of extremists, who remain focused on killing members of the security forces.
Detectives have evidence the dissidents have been actively targeting Police Service of Northern Ireland (PSNI) personnel since the weekend murder.
Officers do not want the nature of the intelligence to be made public for investigative reasons, but they say murder plots are being uncovered at a rate of one a fortnight.