An effort by Yale Security officers to unionize over the summer was blocked by a federal agency after the University registered its opposition.

The officers tried to organize under the American Federation of Labor and Congress of Industrial Organizations, which formally sought approval from the National Labor Relations Board. But Yale’s Office of General Counsel argued that because all the employees seeking to unionize were guards, federal law forbade the specific AFL-CIO union they tried to join from representing them. In July, the NLRB sided with the University.

Now, sources in Yale’s campus safety community said, the setback has not deterred the employees, and they are trying to unionize again, this time with another, independent union. Those sources say the union has strong support among Yale Security employees.

According to the July 16 NLRB ruling, up to 120 Yale Security employees, all of whom work in security operations, could unionize pending federal approval.

At least 30 percent of the employees would need to give their support for unionization before a labor organization can petition the NLRB on their behalf, according to the NLRB website. One campus safety source said a majority of Yale Security employees had originally voted to unionize.

Yale spokesmen Tom Conroy said in an e-mail: “I think it’s best to ask the security guards about their desire to unionize.” University President Richard Levin referred questions to Vice President for Human Resources and Administration Michael Peel, who did not immediately return an e-mail request for comment Monday.

Yale campus safety sources said the heavy restructuring of the security force earlier this year pushed the officers toward unionizing. The University laid off 13 employees in January and appointed former New Haven Police Chief Francisco Ortiz to head security operations.

One Yale Security official said in March that the organizational changes did not treat security employees fairly. Deputy University Secretary Martha Highsmith said at the time that although some employees were unhappy with the changes, the University had tried to be sensitive as it improved and streamlined security operations.

In its ruling, the NLRB explained that a labor organization that already represents non-guards cannot also represent guards. It argued that the Yale Security employees were guards because they constitute an “essential link” in protecting Yale property and community members, even though they do not have authority to make arrests.

The New Britain, Conn., AFL-CIO union Yale Security employees originally tried to join ­­— Council 4 of the American Federation of State, Country and Municipal Employees — represents employees in health care, corrections and law enforcement, public education and other fields, according to its website.