By a slim majority, Yale Security officers voted to unionize on Thursday, but the National Labor Relations Board is investigating four additional ballots, leaving the final outcome unresolved.

The initial vote to unionize passed 66-62, but questions remain about four additional votes. The decision to unionize, which requires a simple majority, has been put on hold because the addition of four ballots could potentially tie the vote.

John Cotter, deputy director for the NLRB’s office in Hartford, said the agent administering the vote challenged the four ballots, over questions of eligibility: the four voters were not on a list earlier supplied to the NLRB, so their ballots were placed in a separate envelope and not looked at.

Cotter said the eligibility of these voters will depend on if they were mistakenly left off the list or if they, in fact, do not satisfy voting requirements such as date of hiring or job classification.

“This situation is uncommon, but not exceptional,” Cotter said, adding that his department faces similar situations a few times a year.

The National Labor Relations Board administered Thursday’s vote from 6 a.m. until around 6 p.m. at both the security headquarters at 100 Church St. and at 79 Howe St.

The vote marks the latest stage in a series of changes at Yale Security over the past year.

The University announced in December 2009 that 13 officers would be let go ­— many of whom were the most experienced and highly paid employees, one former high-ranking Security officer who requested anonymity to avoid retaliation in future employment said. Before the layoffs, this official said there was little talk of unionizing.

The former officer and one current officer, who wished to remain anonymous to avoid impacting his current employment, said the restructuring negatively affected the administration of the department. The former officer said shifts were mismanaged and training interrupted as a result of the lack of institutional knowledge.

These problems and worries of job security, three campus security sources said, were what led some employees to look for a way to organize a union and establish a contract with the University.

Asked about these concerns, Conroy said in an e-mail that, “Yale values [its employees’] expertise and dedication and provides a productive and respectful working environment with excellent salaries and benefits that allow us to attract and retain the best employees.”

He added that the University would not speculate why some Security employees want to unionize.

The first attempt to unionize was this summer under an AFL-CIO union, but Yale contested the move through the NLRB and the federal agency agreed with Yale that the chosen union was not eligible to represent security guards. The employee organizers then contacted SPFPA.

The International Union of Security, Police and Fire Professionals of America, the proposed union for the Yale Security officers, declined to comment until the results of the vote had been announced.

The SPFPA represents over 27,000 security and police officers across the U.S. and Canada, including security departments at Cornell and Princeton universities.

Correction: October 15, 2010

A previous version of this article stated that two campus security sources – who requested anonymity to avoid retaliation in current and future employment – told the News on Thursday that the University was contesting four of the 128 votes. In fact, the NLRB is questioning the eligibility of four additional ballots.