The National Labor Relations Board, the top federal labor law enforcement agency, has dismissed at least one complaint of unfair labor practices filed against the Council of American-Islamic Relations.

Service Employees International Union Local 500 has been trying for about a year to organize the nonprofit Muslim civil rights group's DC staff and has claimed that CAIR is involved in "union-busting," a charge the civil rights group has denied.

In a Nov. 16 letter from the NLRB to SEIU recently made public on the enforcement agency's website, the board's then-acting General Counsel Jennifer Abruzzo said one case of alleged retaliation by CAIR against a staffer involved in union activity was being dropped.

"It was determined that the investigation did not, however, disclose sufficient evidence to establish that [redacted CAIR employee name] was constructively discharged," Abruzzo said.

"Notwithstanding [redacted] protected concerted activities and the [regional office's] merit findings as to the employer’s alleged animus towards this activity, the region’s investigation failed to disclose that [redacted] working conditions were so intolerable as to force [redacted] to resign."

The letter revealed the regional office had rejected the claim in June. The Maryland-based SEIU Local 500 then appealed the dismissal to the full board. Abruzzo's letter was a rejection of that appeal.

It is not clear what the status of the rest of the case is, but the SEIU has said multiple complaints were made, therefore investigations regarding the remaining complaints are presumably still under way.

SEIU Local 500, which has claimed to have the backing of a majority of CAIR staffers, said in April that the civil rights group had fired three staffers involved in the organizing, an effort union spokesman Christopher Honey called "union-busting." CAIR spokesman Ibrahim Hooper said at the time the claim was "meritless."

CAIR has said that as a religious group, it is not covered by the National Labor Relations Act, the federal law that protect workers' rights to form a union. The NLRB's regional director rejected that, stating CAIR was "more akin to a secular civil rights group" and, therefore, its workers were eligible to unionize.

It is not clear how many people are employed by CAIR, which has a Washington headquarters and 30 regional chapters. The union has said the organizing effort involves only the "less than 50" workers at the Washington headquarters.

This is not the first time SEIU Local 500 has bid to represent the staff of a well-known liberal advocacy group and faced resistance from management. In 2014, it bid to represent the staff of Media Matters for America. Media Matters' leadership initially resisted the effort but eventually agreed to let the union represent its staff.

Representatives of CAIR and SEIU Local 500 could not be reached for comment.