Questions have been raised about how dissident republicans obtained enough Semtex to build a major bomb.
The DUP is to request a meeting with police after the stash of the deadly explosive was found during the raid of a house in west Belfast.
More than half-a-kilo of Semtex was discovered, along with two handguns, 200 rounds of ammunition and two detonators.
Unionists have raised concerns that the plastic explosive may have come from an old Provisional IRA arms dump.
The seizure came about when detectives from the PSNI's Serious Crime Branch investigating violent dissident terrorist activity conducted the search in the Ballymurphy Road area in the early hours of yesterday.
The deadly explosive was found in a terraced house close to a school and GAA pitch.
Residents were evacuated from their homes as the property was searched. A 67-year-old man and his 38-year-old daughter were arrested at the scene.
Last night they were continuing to be questioned at the Serious Crime Suite in Antrim, while the items removed from the house have been sent for detailed forensic examination.
Later in the day a pipe bomb was discovered during further searches in Strabane. Army technical officers declared the device to be a viable pipe bomb.
Semtex was synonymous with the Provos' deadly bombs during the Troubles, including the device that killed 12 people during a Remembrance Day service in Enniskillen in 1987.
DUP MP Jeffrey Donaldson said the presence of Semtex raised disturbing questions.
He said his party will be seeking a meeting with police to discuss what was discovered.
"We will be meeting with police about this find because obviously the presence of Semtex is a matter of some concern," he said.
"We do know a number of senior Provisional IRA members who defected to dissident groups had access to arms dumps before they were decommissioned. An amount of Semtex was taken from these arms dumps to dissident organisations.
"I believe one of those involved was the former PIRA quartermaster Michael McKevitt.
"It would seem that some of that Semtex has ended up in west Belfast.
"We will want to ask police if they have any information as to which group they believe was in possession of this arms find."
Ulster Unionist MP Tom Elliott also expressed his concern.
"This has been described as part of an investigation into 'violent dissident terrorist activity', but I note the haul included more than half-a-kilogram of Semtex, handguns and ammunition," he said. "The recovery of Semtex is particularly interesting, given that that particular explosive was brought into this country by the Provisional IRA from Libya several decades ago and was, of course, supposed to have been entirely decommissioned in 2005.
"We need to know how old the material was and if it is proven to have come from an old PIRA arms stockpile, we need to know just how it came into the possession of dissidents."
Sinn Fein MLA Pat Sheehan said: "I condemn whoever is responsible for these weapons and for bringing disrepute to the area. People could have been killed or injured with these weapons. Whoever brought them here has nothing to offer the local community or anyone else."
Detective Chief Inspector Gillian Kearney thanked residents for their patience during the alert. She said: "We are delighted to have removed these potentially lethal items from the streets of west Belfast. We appreciate the clearance operation which was needed to ensure everyone's safety through the night disrupted many people's lives, requiring people to leave their homes and inconveniencing road users.
"We would like to thank them for their patience and understanding during the operation.
"We will continue to work with the community to ensure we keep everyone safe."