Amazon also removed union-related posts on an internet message board and threatened to discipline a JFK8 employee if she re-posted them, the board found. Workers at the warehouse ultimately voted to join the Amazon Labor Union, making it the first and so far only Amazon facility to unionize.

The National Labor Relations Board has ruled it is illegal to force union-backing employees to attend meetings held by their companies that are designed to undermine support and argue against the labor groups.

After a lengthy consideration, the National Labor Relations Board has ruled that Amazon’s “captive-audience meetings” are a violation of the National Labor Relations Act.

The ruling, stemming from a complaint against Amazon, bars companies from compelling workers to attend meetings on unionization’s downsides.