An investigation has begun to establish whether work which began recently on a section of a £160m roads scheme has broken any environment laws.
Contractors moved onto a site at Toomebridge in mid March, which was the subject of a protracted legal battle.
The dispute was over the impact of the A6 road scheme on an overwintering population of protected whooper swans.
The swans are one of the reasons the area has been designated a European Special Protection area.
The area is part of a network of protected nature sites across the EU.
The route of the new road skirts the protected site.
Environmentalist Chris Murphy had made the case that the proposed route would have a negative impact and result in the loss of key foraging grounds.
He claimed the proper environmental assessments had not been carried out - something rejected by judges.
He lost his original case and an appeal in the Northern Ireland courts.
Mr Murphy now intends to try to take it to European courts.
The Department for Infrastructure had given a commitment in its environmental statement that it would not do major earth works between October and mid-March to facilitate the overwintering swans.
Mr Murphy complained to the police and the Northern Ireland Environment Agency (NIEA) last week as equipment moved in.
He said whooper swans were still there in sizeable numbers ahead of their annual migration to Iceland and it would be several more weeks before all had left the site.
The PSNI visited the area and spoke to staff but had left without taking further action.
The NIEA is assessing whether there has been a breach of the Environment Order.
It is likely it will want to establish whether commitments given in the department's environmental statement have been honoured.
"If the presence of a particular species was a qualifying feature of a designated site, there is a possibility of a breach of the legislation used to define the site," a spokesperson said.
"We will not comment further while this investigation is ongoing."
A spokeswoman for the Department for Infrastructure said it was "proceeding with the whole A6 Scheme between Randalstown and Castledawson, taking into account the constraints identified in the Environmental Statement".
"Neither the PSNI nor NIEA have asked the department to cease works", she added.
Mr Murphy said the work would have an impact on the swans.
He said lawyers from the European Commission were now looking at the case.