Five Stormont security guards are facing an Assembly investigation over how an off-duty police officer was able to enter Parliament Buildings in Belfast with a gun.
Staff members involved have been off on sick leave since they were informed by letter last week that a probe was being launched and a representative from their union NIPSA said the security personnel were "extremely upset" at the development.
Security provisions were ramped up at Stormont following an incident in 2006 when convicted loyalist killer Michael Stone stormed Parliament Buildings with an arsenal of weapons in a failed bid to murder senior members of Sinn Fein.
The incident at the end of last month occurred when the police officer passed through the main security check at a search and scanning facility on the driveway up to Parliament Buildings.
The building incorporates bag and body airport-style x-ray machines. The officer, who police said was on a "brief visit" to the building, had a standard issue personal protection weapon on their person.
NIPSA official Noel Griffin said the investigation was in the preliminary stages and had yet to establish if there were grounds for disciplinary action, and claimed that prior to the incident union members expressed concerns about staffing levels in the search facility and rates of sick absenteeism among security personnel.
Mr Griffin said a number of staff also had issues with management, adding in regard to the gun inquiry: "These members will receive full union representation throughout the investigation process."
It is understood agency staff have been drafted in to cover for the staff members that are off sick.
A spokeswoman for the Northern Ireland Assembly stressed there had been no risk to the public or staff as a result of the gun entering the building on Good Friday.