Dissident republican activity has been increasing of late, with police in Northern Ireland fearful of a spate of violent incidents marking the anniversary of the 1916 Easter Rising.
Londonderry's Creggan estate is central to their concerns.
An intelligence-led operation took them into the area late on Thursday night in a hunt for weapons and ammunition.
They were concerned they could be used in the days ahead to attack officers.
The group blamed for killing journalist Lyra McKee is known as the New IRA and was behind a bomb attack outside the city's courthouse at the start of the year.
There have been other signs of violent intentions elsewhere.
Recently, a horizontal mortar tube and command wire were discovered near Castlewellan in County Down.
The dissident republican threat remains classed as severe and in recent days the Police Service of Northern Ireland (PSNI) has been assessing what could, in particular, occur over coming days.
They had called for calm ahead of illegal parades planned in Londonderry and Lurgan in County Armagh.
But that appeal was shattered by gunfire that killed a journalist standing near police lines.