The discovery of two bombs in the past week is evidence that the threat from dissident republicans remains high, Northern Ireland's chief constable has said.
Hugh Orde said there would have been bomb attacks in Northern Ireland but for the actions of the security forces on both sides of the border.
Two people who were arrested in connection with the discovery of a 1,200lb van bomb in Londonderry on Sunday were released without charge on Tuesday.
Two other men are still being questioned by police in Donegal, in the Irish Republic.
On Friday, gardai said they had foiled a major operation by dissident republicans, following the discovery of 500lb of explosives in County Monaghan near the border.
Speaking on Tuesday, Mr Orde said: "The Garda Siochana intercepted one attempt to create a bomb that we think was probably heading to Northern Ireland. "Officers from Derry intercepted another bomb, a huge bomb double the size of Omagh, which we are convinced was also heading for a location in Northern Ireland.
"We don't know where they were going to currently but it shows how high the threat is. It also shows how effective this police service working with the gardai is."
The Derry bomb was found in a van which was being driven suspiciously on the Foyle Bridge at about 0615 BST on Sunday.
Officers found the vehicle abandoned a short time later on the nearby Clooney Road.
Two men are being questioned in County Donegal. They were arrested by gardai in Buncrana.
Two other men are still being questioned by detectives in Derry.
The bomb, which was bigger than the 1998 Omagh bomb, was one of the largest ever found in the UK, according to police.
Explosives, with wires and a timer pack, were found after Army technical officers carried out a controlled explosion on the van.
On Monday, the police said the van had not been stolen but purchased and had been in the possession of the bombers since Friday.
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