Amazon has won a historic warehouse union election. But the battle is not over.

A majority of workers at an Amazon warehouse in Alabama have voted against unionizing with the Retail, Wholesale and Department Store Union (RWDSU) in the first election of its size at an Amazon warehouse in the United States. But the union involved already said before the counting was finished that it would likely challenge the results.

“Our system is broken, Amazon took full advantage of that, and we will be calling on the labor board to hold Amazon accountable for its illegal and egregious behavior during the campaign,” RWDSU president Stuart Appelbaum said in a statement to Recode on Thursday evening.

RWDSU spokesperson Chelsea Connor said part of the alleged behavior involves Amazon’s placement of a USPS mailbox on the grounds of the Bessemer, Alabama, warehouse at the center of the vote. Some workers have said they were intimidated by the installation of the mailbox, as well as the messages from Amazon to use it, and believe that the company wanted to monitor who voted. The Washington Post reported earlier on Thursday that Amazon officials pressed the USPS to install the mailbox, after the National Labor Relations Board (NLRB) denied the company’s request to place a ballot drop box on the property.

Amazon spokesperson Heather Knox previously told the Post that “the RWDSU ... pushed for a mail-only election, which the NLRB’s own data showed would reduce 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 votes against unionizing totaled 1,798, while 738 workers voted to unionize. Another 505 ballots were challenged by either Amazon or the union, but the margin of victory means that the company would still win even if all of the challenged ballots went the union’s way.

“It’s easy to predict the union will say that Amazon won this election because we intimidated employees, but that’s not true,” Amazon said in a blog post. “Our employees heard far more anti-Amazon messages from the union, policymakers, and media outlets than they heard from us. And Amazon didn’t win—our employees made the choice to vote against joining a union.”

Still, the results don’t mean the fight over unionizing this warehouse is over. The union said on Friday morning that it would file “Unfair Labor Practice charges (ULPs) with the National Labor Relations Board (NLRB) charging that Amazon interfered with the right of its Bessemer, Alabama, employees to vote in a free and fair election.” Beyond the mailbox issue, the union called out Amazon’s messaging that discouraged workers from unionizing because of union dues. In Alabama, though, a union cannot force workers to pay dues.

While union organizers believe just getting to a vote of this size at Amazon is a victory in its own right — Applebaum, the union president, said as much on Thursday evening, calling the vote “an important moment for working people” — a loss will still undoubtedly sting. The pandemic exposed wealth and race inequalities in the US that labor activists used as catalysts for their drive in Bessemer, where organizers say at least 80 percent of Amazon employees are Black. If there were a perfect storm to take Amazon on and win, this might have been it.

The RWDSU had already hinted at the grounds for an appeal to the NLRB in recent months, pointing to the mailbox recently installed on the warehouse grounds, as well as Amazon’s message to workers encouraging them to use the mailbox for their ballots.

“I can’t imagine a situation where the workers vote against the union and there’s not a challenge based on that mailbox,” Rebecca Givan, a labor professor at Rutgers University who has followed the Bessemer union drive closely, told Recode before the vote counting began.

If the NLRB rules in favor of the union in its challenge, the board could call for a revote.

Before the vote, labor experts also expected that Amazon would likely challenge the fairness of the election if it lost. Amazon pushed hard for the election to be conducted in person, rather than through mail-in voting, but lost that fight. Lawyers for the company said it wanted as many employees as possible to vote, and argued that NLRB statistics show that mail-in union voting reduces turnout. The company also later lost a request to install cameras in the NLRB room where votes will be tallied, to monitor the ballot boxes during off hours. (Representatives for both Amazon and the union, as well as members of the media, watched the vote counting remotely via Zoom on Thursday and Friday.)

Pro-union Amazon workers were pushing for a seat at the table with management to get more of a say over the demanding pace of work required by the company, as well as the job security, or lack thereof, that comes with Amazon warehouse employment. These workers have said they face constant tracking and surveillance that can be stressful and dehumanizing, as well as what they believe are insufficient break times for the size of the facility, inconsistent timing of breaks during a given shift, and a termination process that can appear one-sided.

While union representatives say union warehouse pay in Alabama averages $18 to $21 an hour compared to an hourly starting wage of $15.30 at Amazon’s Bessemer facility, the topic of pay and benefits isn’t at the top of the list of concerns for many pro-union workers inside Amazon. The company has long said that it didn’t believe the union represented the views of the majority of its workers in Bessemer.

Still, the Bessemer drive might lead to more union pushes elsewhere, whether with the RWDSU or other organizations.

Update, April 9, 2020, 1:28 pm ET: This article has been updated to include Amazon’s comment on the election results.