The complaint stems from charges filed by three Caterpillar workers against the International Association of Machinists (IAM) District Lodge 851 union for allegedly violating their rights and levying retaliatory strike fines against them, according to the National Right to Work Legal Defense Foundation, which has been providing the workers with legal aid.

In May 2012, IAM District Lodge 851 union bosses ordered all 800 workers at the Joliet, Illinois, site on strike.

During the 3.5-month strike, more than 100 workers crossed the picket line, which, according to the union’s constitution, makes them liable to penalties including fines.

After the strike ended, the union levied fines that the foundation claims likely totaled over $1 million, adding that the highest fine reported to them was about $30,000.

Fifty of the workers fined filed federal charges against the union that alleged that they were never truly voluntary union members.

“Although many of those charges have since been withdrawn or settled, the NLRB has issued a complaint for two workers who allege that union officials gave them permission or told them it was acceptable to go back to work and a third worker who resigned union membership before returning to work,” the National Right to Work Legal Defense Foundation said in a statement Thursday.

National Right to Work Foundation president Mark Mix said the aftermath of the strike action proved the need for an Illinois Right to Work law.

“The NLRB will finally prosecute IAM union bosses now that their pattern of workers' rights abuses has become clear," he said.

A hearing before an administrative law judge has been set for August.