Dave Clark, CEO of Amazon’s worldwide consumer business and a top lieutenant of Jeff Bezos, supported an effort to install a mailbox at the site of an Alabama warehouse during a recent high-stakes union election, according to documents presented Monday during a labor board hearing.

The National Labor Relations Board started holding hearings last week to review objections to a failed unionization drive that wrapped up last month at Amazon’s warehouse in Bessemer, Alabama. Bessemer employees overwhelmingly rejected unionization, with fewer than 30% of the votes tallied in favor of joining the Retail, Wholesale and Department Store Union.

The installation of a mailbox outside the Bessemer facility has emerged as a key detail in the RWDSU’s argument that the NLRB should throw out the results of the election. Amazon has contended that it installed the mailbox to make voting easy for employees, while the union alleges that the mailbox may have created the impression of surveillance and sowed confusion among employees by suggesting that the company was involved in collecting and counting votes.

On Monday, the RWDSU presented emails during the hearing that showed Amazon lobbied the U.S. Postal Service to install the mailbox outside of the warehouse.

“Please let me know where we stand on this — this is a highly visible Dave Clark initiative,” Becky Moore, a senior manager of transportation procurement and strategy at Amazon, wrote in an email to USPS officials in January. “Happy to set up a time to discuss tomorrow if needed, but we would like to understand the timeline ahead of us so we can report back up to leadership.”

Jay Smith, director of enterprise accounts for the USPS, testified during the hearing that he never spoke to Clark directly.

In separate emails presented by the RWDSU, Brian Palmer, a senior manager with Amazon’s last-mile public policy team in Washington, D.C., asked Smith whether the company could place a “vote here” sticker above the mailbox’s outgoing mail slot. Smith testified during the hearing that he told Amazon they couldn’t do that.

Smith was later asked about a tent that was placed around the mailbox with a sign that read “Speak for yourself! Mail your ballot here.” Smith testified that he learned from images published in The Washington Post that Amazon had erected a tent around the mailbox.

“I was surprised because I was asked, ‘Can anything physical be put on the box?’” Smith said. “I said no. I did not want to see anything else put around that box indicating that it was a vote.”

The testimony comes after Bessemer employee Kevin Jackson testified last week that he saw Amazon security guards open the mailbox, which was only supposed to be accessible by the USPS. An Amazon spokesperson told Bloomberg that it didn’t have access to outgoing mail and denied any wrongdoing.

The NLRB will review the union’s objections to the election and consider whether they are grounds for overturning the results. The agency could order a new hearing to be held at the facility. Either party can appeal the regional director’s ruling to the NLRB board in Washington.