Stephen Farry, deputy leader of the Alliance Party and MP for North Down, told The Independent it was “too early” to make accusations but to date “answers have not been forthcoming”.
Customs officials carrying out checks on animal-based products at Belfast and Larne ports were asked to stand down from their posts on 1 February following reported threats to their safety.
It came following interventions by Edwin Poots, the Democratic Unionist Party (DUP) minister for Northern Ireland’s Agriculture Department, which is overseeing the new customs arrangements, and Mid East Antrim Council (MEAC).
Mr Poots, who has since stepped away from his post due to health reasons, said he had been contacted by a local official about “threats to staff” at Larne port.
A subsequent Police Service of Northern Ireland (PSNI) probe found “no evidence” of credible threats against workers at either Belfast or Lane ports, although graffiti did appear on a wall near Larne port describing workers there as “targets”.
Police said they had no intelligence suggesting the graffiti was carried out by paramilitaries and was likely the result of a few disgruntled individuals.
“The first and foremost duty on public agencies is to ensure the health and safety of staff, and to err on the side of caution in light of any threats or perceived threats”, Mr Farry said.
“However, there do seem to be major questions regarding the sequence of events and what information from whom was provided to decision-makers.
“It is too early to make accusations around any faults on the part of any individual or wider motivations.
He added: “Rather there is a need for much greater openness and transparency, and an independent investigation may be required. To date, answers have not been sufficiently forthcoming and instead, we have seen considerable deflection.”
MEAC said it removed workers following an “upsurge in sinister and menacing behaviour in recent weeks” after the imposition of the Northern Ireland protocol.
At a meeting prior to the removal of staff on 1 February, MEAC mayor Peter Johnston told councillors: “Trade unions – on behalf of council members of staff assisting with the checks at the port – have raised serious concerns around suspicious activity such as apparent information gathering, including the taking of personal registration plates from their vehicles”.
But unions have since disputed that account.
TUS secretary Alan Law said that while Unite did contact MEAC regarding concerns about graffiti, no union had reported incidents of workers saying they had had their car registration details taken down.
Nipsa, Unite and GMB have written to mayor Johnston calling on him to retract his remarks.
When asked which union had reported concerns about staff car registration details being taken down, a MEAC spokesperson said: “Our threshold for risk when it comes to our staff is very low, and the health, safety and well-being of our workers remains our top priority.
“On Monday 1 February, the mayor of Mid and East Antrim Borough Council was made aware of correspondence to the council’s HR department from a major trade union setting out their concerns.
“The mayor reflected the concerns of the union and highlighted some of the alleged activity which had been reported to the council, from several stakeholders.
“Councillors unanimously agreed to withdraw inspection staff, prioritising their safety pending a threat assessment by the PSNI, which was delivered on Thursday, with staff returning on Friday after Council’s risk assessment.”