FAIRFAX, Va. (AP) — Janitors working at George Mason University who are considering forming a union are facing retaliation from their employer, according to a complaint filed with the National Labor Relations Board.

The complaint filed this week by the union, Service Employees International Union, alleges janitors have been targeted with surveillance and interrogations about potential union activity by their employer, H&E Cleaning in Manassas, Virginia.

The union says the janitors may be improperly classified as independent contractors. The union said in a statement that the janitors are only paid once a month and have received bad checks or checks for less than what they believe they are owed.

The janitors are contract employees and are not directly employed by the university.

A call to H&E for comment was not answered.