Sens. Bernie Sanders and Elizabeth Warren asked Amazon amzn CEO Jeff Bezos to explain himself in an open letter yesterday. They are concerned about reports of “potentially illegal anti-union behavior” at Whole Foods, which Amazon purchased for $13.7 billion last year.

Team leaders at Whole Foods stores recently were shown a 45-minute training video offering advice on discouraging workplace organizing after weeks media reports about workers’ attempts at unionizing. In the video leaked to Gizmodo, the narrator says “We are not anti-union, but we are not neutral either.” The National Labor Relations Act (NLRA) protects employees’ rights to join unions and bargain collectively, and employers are not allowed to threaten adverse consequences or spy on employees.

Sanders began calling out Amazon for the wages it paid warehouse workers in August, noting the contrast between the wealth of Bezos, who recently became the richest man in modern history, with the average worker.

Amazon, including Whole Foods, raised its minimum wage to $15 earlier this month. The senators wrote in the open letter to Bezos: “It is important to recognize that workers’ rights do not stop at the minimum wage, and raising the pay of your lowest-paid workers, while important, does not give you a free pass to engage in potentially illegal anti-union behavior.”

Sanders and Warren asked Amazon to release by Nov. 1 the full video and written script and any other materials distributed to Whole Foods employees regarding organizing activities, as well as what the company is doing to ensure supervisors do not violate federal labor laws, and whether or not Amazon employees were fired or retaliated against for expressing concerns about working conditions.