Amazon is facing questions from Capitol Hill over the safety of its vast delivery network and how the e-commerce giant has evaded responsibility for its role in deaths and serious injuries in crashes involving contractors delivering Amazon packages.

In a letter sent on Thursday to Amazon CEO Jeff Bezos, Sen. Richard Blumenthal decried the company’s “evasive practices and moves to cut regulatory corners,” citing recent investigations by ProPublica and BuzzFeed.

Amazon’s promise of rapid delivery has come with a steep human toll. The ProPublica investigation, which was co-published last week with The New York Times, identified more than 60 crashes since June 2015 involving Amazon delivery contractors that resulted in serious injuries, including 10 deaths.

Amazon has repeatedly said in court that it is not responsible for the actions of its contractors, citing agreements that require them, as one of the documents puts it, to “defend, indemnify and hold harmless Amazon.”

In recent years, Amazon has built a huge logistics operation to get more goods to customers’ homes in less time. To reduce its reliance on legacy carriers like United Parcel Service, the retailer has created a network of contractors across the country that allows the company to expand and shrink the delivery force as needed, while avoiding the costs of taking on permanent employees.

In his letter to Bezos, Blumenthal, a Democrat from Connecticut, demanded information about the contracts Amazon has with third-party delivery companies.

Describing an “aggressive managerial style” that Amazon forces on its delivery companies that has led to a “chain of worker abuse,” Blumenthal called on the Seattle-based retail giant to “immediately cease” business with contractors that violate labor laws.

“It is simply unacceptable for Amazon to turn the other way as drivers are forced into potentially unsafe vehicles and given dangerous workloads,” Blumenthal said in the letter, which was also signed by Sen. Elizabeth Warren, D-Mass., and Sen. Sherrod Brown, D-Ohio.

“The relentless pressure created by Amazon’s delivery policies raises serious concerns about the working conditions its independent contractors and drivers face — creating a system of worker exploitation and abuses.”

An Amazon spokeswoman did not return calls for comment or respond to emails. In an earlier statement to ProPublica and BuzzFeed, Amazon said, “The assertions do not provide an accurate representation of Amazon’s commitment to safety and all the measures we take to ensure millions of packages are delivered to customers without incident.”