A group of Democratic senators sent a letter to Amazon on Thursday questioning it over the firings of four workers who had spoken out about the company’s handling of the coronavirus pandemic.

“In order to understand how the termination of employees that raised concerns about health and safety conditions did not constitute retaliation for whistle-blowing, we are requesting information about Amazon’s policies regarding grounds for employee discipline and termination,” reads the letter signed by the nine lawmakers, including frequent critics of the online retail giant Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.).

Chris Smalls was the first high-profile dismissal during the pandemic. He was fired after organizing a walkout at a Staten Island, N.Y., facility where a worker had tested positive for COVID-19, the disease caused by the coronavirus.

Amazon confirmed this week that another worker at the facility, known as JFK8, died of the disease.

Maren Costa and Emily Cunningham, members of the Amazon Employees for Climate Justice group that protested the company’s climate policies, were fired in early April after criticizing warehouse conditions on Twitter.

Bashir Mohammed was fired the same weekend as the two tech designers after organizing workers at a Minnesota warehouse for more rigorous cleaning and safety.

A spokesperson for Amazon said in a statement to The Hill that “[t]hese individuals were not terminated for talking publicly about working conditions or safety, but rather, for violating—often repeatedly—policies, such as intimidation, physical distancing and more.”