On Friday, six workers died after a tornado demolished an Amazon warehouse in Edwardsville, Illinois.
The next day, OSHA had compliance officers on scene to help in the aftermath, according to Business Insider.
The agency has six months to complete its investigation into safety conditions at the warehouse prior to the roof collapse.
In a statement, Amazon said the company is supporting the investigation.
Retail, Wholesale and Department Store Union President Stuart Appelbaum said it was "inexcusable" for Amazon to require workers to work during a major tornado warning event.
Appelbaum said Amazon "puts its bottom line above the lives of its employees."
Amazon also prohibits its employees from using or carrying cellphones on warehouse floors.
According to Bloomberg, some workers hope to reverse the ban as they could prove useful during emergency events, like a tornado.