The U.S. Department of Labor is asking retail giant Amazon to review its safety procedures in severe weather but isn't levying any fines over a warehouse collapse last December that killed six workers.

In a letter sent to Amazon on Tuesday, the Labor Department said that a number of safety risks were identified during a probe by the Occupational Safety and Health Administration, though the company's severe weather procedures “met minimal safety guidelines for storm sheltering.” The Amazon warehouse collapsed during a tornado in Illinois.

Among other issues, a megaphone to be used in an emergency was locked away, and some employees weren't familiar with where they needed to go in severe weather, since they recalled having no drills on the matter. Amazon's plan for weather emergencies wasn't specific to Edwardsville, Illinois, where the tornado struck.

"The plan was not customized ... and it contained elements that would not be encountered in Edwardsville, such as a hurricane," the letter said

The letter doesn't represent an order to the company.

"I recommend that you voluntarily take the necessary steps to eliminate or materially reduce your employees’ exposure to the risk factors described above," it says.

Though OSHA ruled out legal action in this case, other governmental entities are still investigating the incident. Earlier in April, the House Committee on Oversight and Reform asked Amazon to turn over documents and other information about its emergency protocols.

Safety at warehouses and distribution centers is a hot labor issue. Warehouse workers nationwide have filed formal labor complaints and lawsuits against their employers, alleging they have required employees to work while sick and haven't adequately provided them with protective equipment or enforced social distancing.