Amazon has evaded a request from a group of senators demanding that it disclose the names of the third-party companies it contracts to deliver packages all across the United States.

In September, Sen. Richard Blumenthal (D-CT) along with Sens. Elizabeth Warren (D-MA) and Sherrod Brown (D-OH) penned a letter to Amazon CEO Jeff Bezos. The letter was in response to a report by BuzzFeed News and ProPublica that uncovered the company’s use of contractors to deliver its packages, with a focus on the poor treatment and conditions those workers face in addition to the lack of adequate workplace protection and benefits. Since these companies are not owned by Amazon, the company can deny all responsibility.

In its response letter, Amazon claims that it regularly audits its contractors for labor safety violations and provides contract employees a “secure network” for raising complaints. “As a result of information received through the reporting network, we have terminated contracts with DSPs,” Amazon wrote. However, it did not disclose the names of any of the over 800 delivery providers it hires.

“I’m deeply disappointed by Amazon’s evasive response”

“I’m deeply disappointed by Amazon’s evasive response,” Blumenthal said in a statement. “Amazon’s claims that it regularly audits and sanctions delivery service providers ring hollow given the troubling facts laid out in these reports. At a minimum, Amazon is falling far short of its responsibility to ensure its contractors are following labor laws and safety regulations.”

Amazon also refused to directly respond to questions the senators posed alleging that the company has worked to block the creation of a union. In its response letter, Amazon wrote that it “respects its employees’ right, and those of our delivery providers, to choose to join or not join a labor union.”

Over the past few years, lawmakers and labor activists have taken aim at Amazon over its labor practices. Last year, Sen. Bernie Sanders (D-VT) introduced a bill that scared Amazon into paying its employees $15 an hour, called the “STOP BEZOS Act,” and reports have surfaced alleging the company has failed to pay its workers overtime. The company has also engaged in aggressive anti-union tactics over the years, particularly with regard to warehouse workers, and has faced protests worldwide over working conditions.

“Amazon chooses to hide behind its delivery partners and a mandated veil of secrecy to protect itself from a disastrous record and true accountability,” Blumenthal 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.”