The National Labor Relations Board is investigating a pair of allegations that U.S. Sen. Bernie Sanders' Iowa campaign cracked down on union activity and "refused to bargain in good faith," according to federal filings.

The federal agency redacted the name and other identifying information of the person or people who made the allegations of unfair labor practice. It assigned a field attorney to investigate the claims and decide if they warrant a hearing with an administrative law judge.

The allegations include that campaign officials fired one or more employees because they joined or supported unions or engaged in other protected activities, such as discussing wages and other conditions of employment.

In the past decade, about 1 in 20 allegations of unfair labor practices reach the need for a hearing, according to the NLRB. The process can take months.

Just because the complaint to the federal board identified Sanders' Iowa campaign does not mean the petitioners work for the presidential campaign, according to the agency's process. The documents filed with the federal agency say that the Sanders campaign "told us we were 'at-will' in spite" of a collective bargaining agreement.

The Sanders campaign unionized in March and was the first presidential campaign to do so.

The campaign and the union ratified a contract in May, although disputes about pay lingered with some workers claiming they were not making at least $15 an hour. In a July interview with the Des Moines Register, Sanders said the campaign would cut back on hours to make sure all workers earned the hourly standard. Campaign officials have since given those who were not making $15 an hour a raise so they reach that minimum.

► More: Bernie Sanders defends staff compensation after complaints his campaign isn't paying $15 an hour

Officials from the Sanders campaign and the United Food and Commercial Workers Union, which represents his campaign workers, said they were unable to comment on the specific allegations without compromising the board's process.

“We are committed to cooperating with the NLRB and we are confident that they will find the campaign honors all of its employees’ rights to both the letter and spirit of the law. Sen. Sanders and the campaign believe all workers should have a strong voice on the job and the right to due process, including the right to petition to the NLRB," campaign officials said in a statement provided to the Register.

Nick Coltrain is a politics and data reporter for the Register. Reach him at ncoltrain@registermedia.com or at 515-284-8361. Your subscription makes work like this possible. Subscribe today at DesMoinesRegister.com/Deal.