Amazon has refused a request from three U.S. senators to disclose the names of the companies that deliver millions of packages to homes across the country, providing what one lawmaker called “evasive” responses to questions about the e-commerce giant’s network of delivery contractors.

Last month, Senators Richard Blumenthal, Elizabeth Warren and Sherrod Brown demanded information from Amazon CEO Jeff Bezos after the company’s delivery network was the subject of investigations by BuzzFeed News and ProPublica. Those reports found that Amazon uses contractors to carry out an increasingly large share of its deliveries and that the system has led to worker abuses and jeopardized public safety. When problems arise, Amazon denies responsibility, saying it can’t be held to account for the actions of independent contractors, though the company keeps a tight grip on how the drivers do their jobs. At least 10 people have died in crashes involving Amazon delivery providers, ProPublica found.

Now in a letter responding to those senators, Amazon said that safety is its “top priority” and that it sets “standards that meet and often exceed legal requirements.” Amazon sent the letter late Friday and the senators shared it with BuzzFeed News and ProPublica this week.

Saying he was “deeply disappointed by Amazon’s evasive response,” Blumenthal added that it gives him “no confidence that the company is committed to preventing the types of tragedies chronicled by BuzzFeed and ProPublica.”

“At a minimum,” Blumenthal said, “Amazon is falling far short of its responsibility to ensure its contractors are following labor laws and regulations.”

The letter said Amazon runs background checks on companies that apply to deliver its packages and regularly audits its more than 800 delivery contractors for “compliance with legal and safety requirements.” In declining the request from the senators to reveal the names of those companies, it called that data “competitive, confidential business information.”

The company also sidestepped a question from the senators about whether it has attempted to “thwart the creation of a union.” Amazon said only that it “respects its employees’ right, and those of our delivery providers, to choose to join or not join a labor union.” The letter did not address reports that Amazon executives had met with delivery contractors in Chicago in 2017 to advise them on how to prevent organizing, and that it had held a similar meeting with delivery contractors in Canada. The only Amazon delivery provider that successfully organized lost its routes and ceased operating in that state soon thereafter, the BuzzFeed News report found.

The U.S. Department of Labor has uncovered numerous employee abuses at firms delivering for Amazon, including shorting worker pay and failing to pay overtime. In its letter, Amazon said it requires its delivery contractors to offer a host of benefits and “provide their employees with competitive wages of at least $15 an hour.”

Yet a review of online job listings on Wednesday turned up at least two Amazon delivery contractors — one of which has contracts with the e-commerce firm in more than a dozen states — seeking to pay drivers $14 an hour in some areas. Seven others advertised delivery jobs with wage ranges starting as low as $13 an hour, although some of those job postings said they could go as high as $16 an hour.

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Amazon, which also relies on UPS and the U.S. Postal Service to deliver packages, began creating its own delivery network in 2014 and has since contracted with hundreds of firms — many of them operating unmarked white vans — to carry boxes and envelopes to their final destination.

As BuzzFeed News reported last month, some of those contractors have extensive histories of financial disarray, serious crashes and labor law violations. Although Amazon said in its letter that it “terminates contracts” with firms that have repeated labor violations, federal records show several companies still under contract that have been sanctioned multiple times by the Labor Department.

In the letter to the senators, Amazon appears to ignore the existence of such contractors, referring only to the latest iteration of its delivery network, rolled out just last year and featuring dark gray or blue Amazon-branded vans.

Amazon notes that drivers for its delivery firms receive four days of safety training before being put on the road, but it is unclear when Amazon began providing that training, and it is substantially less than what UPS provides its drivers. At least one of the new generation of Amazon firms, based in Southern California, has already been sued by a driver who claimed she was denied mandatory meal and rest periods and sick leave, and was not paid correctly, court records show.

“Amazon’s response gives me no confidence that the company is going to take full responsibility for the delivery of its packages and make sure all contracted companies are complying with labor law,” Brown, an Ohio Democrat, said in a statement. “Amazon must make the safety and the working conditions of all of its delivery drivers a top priority. I will continue to pressure them until they do so.”

A spokeswoman for Warren, a Democrat representing Massachusetts and a candidate for president, said that “Amazon should be focused on getting to the bottom of these troubling reports and learning how it can do better by its third-party contract workers, not on trying to lobby Congress to look the other way.”

Amazon would not provide additional comment beyond the contents of the letter.

It did, however, invite the three senators to “tour any of our facilities” in order to “see first-hand how we are committed to safe operations for our customers, employees, and third party delivery partners across our network.”

Blumenthal did not seem inclined to take the company up on its offer.

“If Amazon were committed to proving its safety record, it would throw open its books and invite real scrutiny into its relationships with third-party delivery companies,” the Connecticut Democrat said. “Amazon’s time would be better spent working to provide specific answers to the questions we asked and addressing the troubling issues we raised.”

Here’s the full text of Amazon’s letter.