A federal agency has referred a union’s organizing drive at Delta Air Lines to the Justice Department over questions about fraudulent signatures.

The International Association of Machinists and Aerospace Workers earlier dropped a request for an election to decide whether it would represent Delta flight attendants.

On Thursday, the National Mediation Board, which oversees elections in the airline industry, said the integrity of its election process “has not been respected in this case.”

Board General Counsel Mary L. Johnson said in a letter to Delta and the union that the board had reason to believe that somebody turned in election-authorization cards with fraudulent signatures in possible violation of federal law. Johnson said the board referred the matter to the Justice Department for further review.

A spokesman for the union did not immediately respond for comment.

Atlanta-based Delta, the nation’s third-biggest airline company, has claimed that flight attendants questioned the validity of some authorization cards. The airline issued a statement Thursday from CEO Richard Anderson, who said he supported a Justice Department investigation “so our people can know the full truth.”

Only 18 percent of Delta Air Lines Inc. employees are represented by a union, less than one-fourth the percentage at the other three leading U.S. airlines, according to regulatory filings by the companies. The machinists union said earlier this week it would try again next year to organize the Delta flight attendants.

Photo Credit: Delta Air Lines CEO Richard Anderson talks during an interview, in New York. Mark Lennihan / Associated Press