Police in Northern Ireland said on Tuesday they had recovered four suspected pipe bombs from a cemetery near the city of Londonderry following a public safety operation.
The discovery comes hours before US President Joe Biden is due to arrive in Belfast, about 110 kilometers (70 miles) southeast of Londonderry.
On Monday, police said a number of masked people attacked a police vehicle with petrol bombs and other objects at a parade opposing the Good Friday peace accord in Londonderry. The devices were found in an area linked to that parade, police said.
"The discovery of these devices was a further sinister and worrying development," Assistant Chief Constable Bobby Singleton said in a statement.
Biden arrives in Belfast late on Tuesday at a delicate political time in Northern Ireland as he helps mark the 25-year anniversary of a peace deal that largely ended 30 years of bloodshed there.
What is a pipe bomb?
Pipe bombs are crude improvised explosive devices used in the past by militants in Northern Ireland and are often around the size of a hand grenade. They are made by tightly filling a section of pipe with explosives.
Police said the operation in the cemetery is now complete and police officers have left the scene. The devices were discovered at 0530 GMT.
"Colleagues from Terrorism Investigation Unit will now lead on this investigation," Singleton said.
In March, Britain's MI5 intelligence agency increased the threat level in Northern Ireland from domestic terrorism to "severe" - meaning an attack is highly likely - though the move was not thought to be linked to the anniversary.