Their paychecks are signed by hundreds of companies, but often Amazon directs, through an app, the order of the deliveries and the route to each destination. Amazon software tracks drivers’ progress, and a dispatcher in an Amazon warehouse can call them if they fall behind schedule. Amazon requires that 999 out of 1,000 deliveries arrive on time, according to work orders obtained from contractors with drivers in eight states.

Amazon has repeatedly said in court that it is not responsible for the actions of its contractors, citing agreements that require them, as one puts it, to “defend, indemnify and hold harmless Amazon.” Just last week, an operations manager for Amazon testified in Chicago that it signs such agreements with all its “delivery service partners,” who assume the liability and the responsibility for legal costs. The agreements cover “all loss or damage to personal property or bodily harm including death.”

Amazon vigilantly enforces the terms of those agreements. In New Jersey, when a contractor’s insurer failed to pay Amazon’s legal bills in a suit brought by a physician injured in a crash, Amazon sued to force the insurer to pick up the tab. In California, the company sued contractors, telling courts that any damages arising from crashes there should be billed to the delivery companies.

“I think anyone who thinks about Amazon has very conflicted feelings,” said Tim Hauck, whose sister, Stacey Hayes Curry, was killed last year by a driver delivering Amazon packages in a San Diego office park. “It’s sure nice to get something in two days for free. You’re always impressed with that side of it. But this idea that they’ve walled themselves off from responsibility is disturbing.”

“You’ve got this wonderful convenience with this technology,” he added, “but there’s a human cost to it.”

Amazon, the world’s largest retailer, is famously secretive about details of its operations, including the scale of its delivery network. In many of the accidents involving its contractors, drivers were using cars, trucks and cargo vans that bore no hint of Amazon’s corporate logo. The truck involved in Gabrielle Kennedy’s death, for example, was marked only “Penske Truck Rental.”

Amazon declined to answer questions about the demands it places on drivers, the anonymity of delivery vehicles or any requirement that these contractors indemnify Amazon.