Amazon.com Inc. should better prepare workers for extreme weather events, according to federal regulators who investigated a deadly tornado strike on a company warehouse in Edwardsville, Illinois.

The storm ripped through the facility in December, killing six workers and injuring several others, prompting the Occupational Safety and Health Administration to launch a probe. At the time, Amazon said the facility complied with all construction regulations and that proper safety procedures were followed when the tornado struck. But several workers told Bloomberg that training for such events was minimal and mostly entailed pointing out emergency exits and assembly points.

An OSHA report released on Tuesday echoed those concerns. The agency said a bullhorn that was supposed to be used to tell workers to take cover was locked up in a cage and inaccessible. In interviews with investigators, some employees couldn’t recall ever participating in emergency drills and said they mistakenly took shelter in a bathroom on the south side of the building rather than in designated restrooms on the north side.