Amazon built its own in-house mobile app called Mentor that it uses to track the location and rate the performance of third-party delivery drivers, according to a new report from CNBC. The app tracks a driver’s location at all times and also generates a daily score that factors into performance evaluations, with low scores potentially harming a third-party delivery company’s relationship with Amazon for future contracts, the report states.

Amazon bills the app as a tool for improving safety, but both drivers and privacy proponents cited by CNBC worry the Mentor software is also a tool for surveillance of drivers and as another form of pressure applied on the workers to ensure they’re delivering packages as fast as possible. Earlier this month, an Amazon promotional video posted online revealed a new initiative involving always-on security cameras from a firm called Netradyne that are similarly used to monitor and measure a driver’s performance, causing alarm among privacy advocates.

“Safety is Amazon’s top priority. Whether it’s state-of-the art telemetrics and advanced safety technology in last-mile vans, driver-safety training programs, or continuous improvements within our mapping and routing technology, we have invested tens of millions of dollars in safety mechanisms across our network, and regularly communicate safety best practices to drivers,” an Amazon spokesperson told CNBC.

As its operations have expanded, Amazon has increasingly built out its own delivery infrastructure, part of which is the delivery service program (DPS). This program contracts third-party companies to help with the last-mile delivery that, until a few years ago, was mostly handled by companies like FedEx and UPS. Amazon uses its own drivers to help with cost-cutting measures and to increase the speed and efficiency of deliveries, as part of its broader effort to own and operate more of its logistics chain with the purchases and leases of cargo freighters and airplanes.

There have, however, been issues regarding driver and pedestrian safety. Amazon has, in some cases, skirted direct accountability for crashes, injuries, and even deaths at the hands of drivers delivering its packages to customers’ homes because of the contract nature of the work, according to The New York Times. Still, the company has come under fire for using software and other tools to prioritize the speed of deliveries over potential safety concerns.

As a result, Amazon has reportedly begun exercising increasing control over its drivers by managing their routes and monitoring their behavior at all times with the goal of improving safety. The Monitor app and always-on surveillance cameras are part of those measures, though some drivers are concerned about how invasive these methods are.

CNBC says drivers congregate online and discuss ways to try to game the system. For instance, drivers have tried wrapping their phones in sweaters and storing them in the glove box of delivery vans because of how unfairly they’re dinged for standard procedures on the job that the Monitor app mistakes for reckless driving, like marking a delivery as completed by picking the phone up. “If your device moves at all, it’s going to count against you,” one driver told CNBC.

There are also concerns about the location sharing permissions. Drivers are asked to download the app on their personal devices and to enable “always allow” location sharing, which has some drivers concerned Amazon is overstepping its boundary with regard to driver privacy, CNBC reported.