Police in the British province said on Saturday that examination of several pipe bombs used against Catholic targets in recent weeks points to the Ulster Volunteer Force.
The attacks against members of Sinn Fein, the political wing of the Irish Republican Army, have not caused any injuries and no-one has claimed responsibility for the blasts. Sinn Fein favours the province's merger with the Republic of Ireland.
In the most recent incident, a device was discovered lying under a van belonging to Sinn Fein member Michael Agnew in the mainly Protestant town of Ballymena.
Two men have been detained, and police may hold them up to a week under anti-terrorist legislation.
The Ulster Volunteer Force is an outlawed group formed in 1966 to defend the province's status as part of the United Kingdom. It was reported to have carried out bombings, assassinations, kidnappings, extortion, and robberies before it declared a ceasefire in 1994.
The UVF was one of the parties which signed the historic Good Friday Agreement in 1998.
Police said the bomb found under Agnew's vehicle, composed of plumbing pipes stuffed with explosives and metal fragments, was a sophisticated device, similar to other bombs discovered three weeks ago under the car and in front of the house of another Sinn Fein member in Ballymena.
"The Ulster Volunteer Force has breached its ceasefire and gone back to war," said Sean Farren, a representative of the mainstream SDLP Irish nationalist party.
But David Ervine, a spokesman for the Progressive Unionist party, which is close to the UVF, denied the accusation.
"The UVF in Ballymena are not responsible. I'm convinced of that," he said.