U.S. regulators are calling on Amazon to improve its procedures for dealing with severe weather like hurricanes and tornadoes that could threaten workers at its warehouses dotted across the country.

The Occupational Safety and Health Administration on Tuesday sent a “Hazard Alert Letter” to the Seattle-based e-commerce giant on Tuesday following the agency’s investigation into the deadly collapse of a company warehouse in Edwardsville, Illinois in December. Six people died and another was critically injured in the tornado strike.

The investigation raised concerns about the potential risk to employees during severe weather emergencies,” according to the letter sent to Amazon that OSHA made public.

The agency said its inspection found that, while the company’s severe weather procedures had met minimal federal safety guidelines for storm sheltering, the company still needed to further protect its workers and contract employees. The letter requires Amazon to review its severe weather emergency procedures but the company won’t face any fines or penalties.

In interviews with Amazon and contract workers, OSHA officials found some employees couldn’t recall ever participating in severe weather drills, or the location of the facility’s shelter. Kelly Nantel, an Amazon spokesperson, however, said employees receive emergency response training, which is “reinforced throughout the year.”

“OSHA’s investigation did not find any violations or causes for citations, but we’re constantly looking to innovate and improve our safety measures and have already begun conducting additional safety and emergency preparedness drills at our sites and will carefully consider any OSHA recommendation that we have not already,” Nantel said.

Amazon has noted workers at the warehouse, known as a “delivery station,” had little time to prepare when the National Weather Service declared a tornado warning on December 10. About 10 minutes before the tornado touched down, the agency said managers directed workers to go to a restroom in response to tornado warnings and other weather alerts.

But some employees unaware of the designated tornado shelter — a restroom located in the northern portion of the building — went to a separate restroom in the hard-hit south end, the agency said. All the injured and killed had taken shelter in the south side bathroom.

John Felton, Amazon’s senior vice president of global delivery services, had said in December that most of the 46 people in the warehouse headed to a shelter on the north side while a smaller group went to the south end, where the loading docks were located and delivery vehicles were parked.

“The tornado that hit our delivery station was extreme and very sudden, with winds that were much like the force of a category 4 hurricane, and we believe our team did the right thing, moving people to shelter as soon as the warning was issued,” Nantel said.

In the inspection, the agency said it also reviewed contractor safety and training records as well as the facility’s written “Emergency Action Plan.” Officials took issue with the plan, writing in the letter it “was not customized with specific instructions” for hazards expected at the Edwardsville site.

Though Amazon had posted evacuation maps at the facility showing the location of the designated shelter, officials found the written plan did not “specifically identify” the shelter’s location in the warehouse. Nantel said the company’s buildings have “emergency plans that identify exit routes and shelter areas.”

Separately, officials said a megaphone that was supposed to activate the site’s shelter-in-place procedure was locked in a cage and not accessible, resulting in managers verbally telling workers on where to take shelter.

“These tragic deaths have sparked discussions nationwide on the vital need for comprehensive workplace emergency plans,” William Donovan, OSHA’s regional administrator in Chicago, said in a statement. “Employers should re-evaluate their emergency plans for the safest shelter-in-place locations and prepare before an emergency to ensure workers know where to go and how to keep themselves safe in the event of a disaster.”

The agency has recommended Amazon makes its warning devices readily accessible, ensures all employees participate in drills for emergency weather events and include site-specific information in its emergency plans. It said it will also send hazard alert letters to three delivery service providers, who employed the injured worker and five of the employees who died.

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