The mailbox is located near the entrance of the Amazon factory and has become an important evidence for the union’s bid to overturn the election results.

A worker said at a hearing of the National Labor Relations Board on Friday that security guards at Amazon.com Inc. warehouses in Alabama had a key to a mailbox, and the company encouraged employees to hold it earlier this year. Used this mailbox to mail ballots in the high-profile union elections.

Kevin Jackson, who has worked in the Bessemer warehouse for more than a year, said he saw two guards approaching the mailbox and using a key to open one of the doors. The mailbox is located near the entrance of the Amazon factory and has become an important evidence for the union’s bid to overturn the election results.

In a complaint on April 16, the Retail Wholesale and Department Store Alliance accused Amazon of misbehaving, including issuing anti-union threats, firing an employee who issued a union card, and forcing workers to vote using mailboxes.

The mailbox was provided by Amazon at the request of the U.S. Postal Service, and union officials accused the company of installing the mailbox to monitor workers’ voting. Federal law restricts the monitoring of election activities of employee unions. The union also claimed that the installation of mailboxes gave the impression that Amazon “controls the election mechanism” according to the order of the Acting Regional Director of the Labor Bureau on April 30.

Amazon usually refuses to ask for the ulterior motives of the mailbox, saying that its installation is to ensure a strong turnout. The company said in a statement last month: “This mailbox, which is only accessible by USPS, is a simple, secure and completely optional way for employees to vote easily, no more or less.”

Amazon did not immediately respond to a request for comment on Jackson’s allegations.

The “cluster” mailbox has several locked doors. Jackson said that when he got off work one morning, he saw security guards approaching the box, and then one of them used a key to open the large box marked “1P” at the bottom. He said: “I’m not sure what he is going out or looking for.”

Amazon employees voted 1,798 to 738 against joining the union in a seven-week mail-in election that ended in March. The labor committee has the power to invalidate the election results in order to respond to behaviors that may change the election results and prevent employees from freely choosing whether to join the union.