(Bloomberg) -- A mailbox near Amazon.com Inc.’s warehouse in Bessemer, Alabama, has emerged as a key piece of evidence in an effort to overturn the results of a union election.During a National Labor Relations Board hearing on Friday, an employee said Amazon security guards used keys to open the mailbox, which the company says was installed so workers could more easily mail their ballots.The Retail, Wholesale and Department Store Union, which lost the election last month, has accused Amazon of locating the mailbox near the facility’s entrance so it could spy on workers. Even just the impression of election surveillance by an employer can influence the vote’s outcome, so the hearing is focusing largely on what workers witnessed during the vote.Amazon has denied an ulterior motive, saying it asked the U.S. Postal Service to install the mailbox in a bid to boost turnout.“This mailbox -- which only the USPS had access to -- was a simple, secure, and completely optional way to make it easy for employees to vote, no more and no less,” the company said in a statement last month.But the employee, Kevin Jackson, seemed to contradict Amazon’s assertion that only the USPS had access to the mailbox. Jackson testified that as he was leaving work one morning he saw security guards approach the mailbox and then watched one of them open a large box on the bottom labeled “1P.”“What he was getting out or looking for, I’m not sure,” he said.While it was dark at the time, Jackson said his headlights illuminated the area. He also testified that Amazon security guards used surveillance cameras to monitor the parking lot, including zooming in on particular cars to read license plates and see who was inside if they detected anything suspicious.Amazon didn’t immediately respond to a request for comment on Jackson’s testimony.If the board finds Jackson’s testimony credible, it should be reason enough to overturn the election result, said former NLRB member Wilma Liebman, who chaired the board under President Barack Obama. What Jackson is alleging “at a minimum creates an appearance of Amazon involvement -- if not an inference of actual tampering -- with ballots,” Liebman said via email Friday.Under long-standing precedent, NLRB elections are supposed to be held under “laboratory conditions,” in which “employees are able to exercise free and uncoerced choice” about unionization, Liebman added. “This conduct certainly departs from that.”“What legitimate purpose could there possibly be for Amazon security guards to be opening the box?” she asked.In an April 16 complaint, the union accused Amazon of misconduct -- including issuing anti-union threats, firing an employee for distributing union cards and pressuring workers to use the mailbox to cast their votes. Amazon workers have been testifying all week about the mailbox and how the company encouraged them to use it as a secure way to submit their ballots.A labor board hearing officer who will recommend a ruling on the matter asked Jackson several questions about what he saw, indicating she deemed his testimony potentially relevant to the case.Like other Amazon employees who testified this week, Jackson said he was approached by managers asking him if he received a ballot and whether he voted.One Amazon manager, whose name Jackson didn’t know, asked him “off the record” what he felt about the union campaign. Jackson said he responded by quoting scripture. The manager, Jackson said, told him, “That’s not the answer I was looking for,” and asked him again. Jackson said he repeated the same Bible verse.“She was frustrated with my answer and walked off,” he said.Jackson, who was recently hospitalized for an undisclosed health condition, testified via videoconference while lying down. Amazon attorneys asked if he was taking any medication that would affect his memory and he responded that he wasn’t.Amazon employees voted 1,798 to 738 against joining the union, with 505 other disputed ballots uncounted. The labor board has the authority to invalidate election results in response to conduct that could have changed the outcome and prevented employees from making a free choice about whether to unionize.“This damning eyewitness testimony of an Amazon security guard opening the famous mailbox (to which the company insisted only USPS had keys) comes as no surprise,” said Representative Andy Levin, Democrat of Michigan. “But the worker’s riveting account of a supervisor asking him ‘off the record’ (right!) whether he was for the union is just as important. As I have suspected since my first visit to Bessemer, Amazon broke the law, and the NLRB will likely come to that conclusion.”(Updated with comments.)For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2021 Bloomberg L.P.