The US House Oversight Committee has opened an investigation into shipping giant Amazon's practices during extreme weather events. The probe was launched largely as a response to the company's handling of employee safety during a swarm of tornadoes last year that left six workers dead.

The committee sent a letter to Amazon Chief Executive Andy Jassy on Thursday asking Amazon to provide policies detailing emergency preparation and any relating to the tornadoes that destroyed an Amazon facility and killed workers in Edwardsville, Illinois, in December.

"The Committee seeks to fully understand the events that led to the tragedy at Amazon's Edwardsville facility," the committee wrote.

The letter was signed by Democratic Representatives Carolyn Maloney, Alexandria Ocasio-Cortez and Cori Bush.

"We are concerned by recent reports that Amazon may be putting the health and safety of its workers at risk, including by requiring them to work in dangerous conditions during tornadoes, hurricanes, and other extreme weather," the lawmakers wrote. “As one of our country’s largest and most profitable corporations, it is imperative that Amazon protect workers’ safety and refrain from practices that could put them in danger.”

Questions over Amazon's extreme weather preparedness and leave policies arose after the tornadoes hit the Edwardsville facility. NBC News reports that at least five workers reached out to journalists claiming their superiors has warned them they would be fired if they left their shifts early to find shelter.

The family of one of the victims who died at the facility has launched a wrongful death lawsuit against Amazon since the incident.

"We also seek information about Amazon’s workplace policies or practices that may have prevented the workers from seeking safe shelter, as well as Amazon’s actions in responding to other severe weather incidents and natural disasters," the letter said.

Kelly Nantel, an Amazon spokesperson, said in a statement that "our focus continues to be on supporting our employees and partners, the families who lost loved ones, the surrounding community, and all those affected by the tornadoes. We will respond to this letter in due course."

The lawmakers will also examine claims of disciplinary action taken by the company against workers or contractors at facilities where employees were required to remain on the job during the wildfires in California in 2018, extreme heat events in the Pacific Northwest, and flooding during Hurricanes Irma and Ida in 2017 and 2021, respectively.

The blowback against Amazon by concerned workers came to a head on Friday, when workers at a Staten Island facility voted to form a union, the first ever to be established within the company. Employees voted 2,654 for the union and 2,131 against. The victory has been hailed by labour analysts as one of the most significant labour victories in decades.